Theories I think are basically canon

The shadow men in chapter 3 of Deltarune are the shadows in the window of Kris' living room. Maybe old action figures?

Friend is not a cat, it's a lot of different cats smushed together. (Personal note: maybe this is why there's a face on cat Mike’s tail- to show there's multiple cats?). That's why it has five legs. It also is visually similar to undertale’s endogeny, which is an amalgamation of dogs. You also multiple cats you fight at once in chapter 3's sword route

Gaster is a skeleton. This is probably true because the mystery man sprite (most likely candidate for Gaster's physical form) very much looks like one, and he is both named after and speaks in a type of font- in a game where the only two other characters who are named after and speak in a font are skeletons.

Friend is some sort of demon, or an amalgamation of demons. This is becuase in Undertale's code there's unused lines called Demon a through demon d. A smile in a vending macine in chapter 3 then directly quotes the 'demon D' line about being your upmost servant. (We know this is friend in the vending machine because the smile is one of his's most iconic features, the vending machine lies about the smile being free- it's actually a dollar- which lines up with the pattern of friend hurting you but also having the possibility to help you out whenever he shows up (In the mic minigame he can either heal or hurt your health- though his damage takes off more points than his healing adds, when attacking you in sword route in chapter 3 he drops candy that heals you when defeated. In chapter 5 when he harms you on the platforms you immediately fall to an area with an item that allows you to heal?), and the vending machine is also right outside the sword route room where you use a pink and gold controller and Friend literally appears). There's no other reason to directly quote the demon D line in Friend's dialouge, especially when we already know he's an antagonist, if this weren't the case. Also, the fact he typically helps you a little whenever he hurts you lines up extremely well with the idea of making a deal with the devil.

Gaster is not a good person. His stats are all 666 plus or minus a few 6's, the sprite assumed to be him appears with a fun value of 66, and the river man tells you to beware him. (Also, Sans' Gaster Blasters are weapons). Plus, if it's assumed that he's the one who created Deltarune, then he added Friend (assumed to be a demon) to the game under the name. Uh, "Friend"- he's calling a demon his friend.

Friend is Mike. This is cause the door labeled "Mike" in chapter 3 literally has his face on it, and when you ask Spamton about friends he goes 'we don't need mike'

Random stuff lol

In the Spamton sweepstakes, the egg in Noelle's game left home due to happiness, after, instead of being disappointed that the special EGG never hatched and was lowkey creepy, she spent time treating it like a normal pet and brushing it and giving it affection and whatnot. Typically the pets leave due to sadness.

Gaster is associated with "fun value". Friend's name is. Well. "Friend", and one of his most iconic features is that he is constantly smiling (personal note: I think this- the devil being associated with smiles and fun and joy- is just a motif Toby likes. We also know Toby likes demons/hell motifs in general because his message to data miners in Undertale's code went under the names DemonX, DemonY and DemonZ))

In the true lab in Undertale, Alphys and Gaster were conducting Determination expiriments on Monsters who had fallen down (a coma like state monsters enter before they die) to try and revive them. In entry in the true lab, the author says they thought the fallen down monsters- who they managed to revive- were goners. Gaster's followers are called Goners. Gaster's followers are also all monsters, which we know because- beside their non human appearances- they're monsters in Deltarune

Friend is a cat, and Kris is repeatedly associated with dogs

There's no cats in undertale, but a lot of dog-like creatures. Bratty even calls this out in the true pacifist route epilouge: "[Bratty] Anyway, when was the last time you even SAW a cat around here?" (from the wiki)

The riverman mentions a song from the sea in Undertale. Onion mentions hearing a song coming from the sea in Deltarune

Memoryhead's overworld sprite in Undertale has, in it's center, a thing that looks identical to what the fandom has dubbed 'Eram' in Deltarune. Memoryhead's thing has eyes but no mouth, Eram has a mouth but no eyes

Whatever the heck this is, which refrences Deltarune's theme song- don't forget.

The demon shit

When the first demo of Deltarune was released (called SURVEY_PROGRAM), "the file version was 0.6.6.6 also, you had to sign an agreement before starting the game that said in all caps you accept everything that will happen from now on" Deltarune Cat and Mouse Theory by cammyboig (personal note: seems like a deal with a demon to me) *

There are Four lines of text in Undertale that you can access only by digging through the files, called "demon A" through "demon D"

• "Greetings"
• "You have made yourself completely clear"
• "Understood"
• "I, your humble servant, will follow you to the upmost"

In Deltarune, Gaster's unused battle stats hidden in the files (attack, defense, stats) are all either 666, 6666, 66666, or 666666

In Deltarune, the fun value that allows you to see the mystery man (most likely candidate for Gaster's physical form) is fun value 66 because of course it is.

Kris practiced the occult with Catty and tried to summon a demon

Gaster's expiriment that he fell into was Deltarune, or at least the darkworlds. Gaster is very conected with the meta of both Undertale and Deltarune; info on him is best found through digging through Undertale's files, Toby announced Deltarune by speaking like Gaster on Twitter/it's implied Gaster's the one who created Deltarune, Gaster's the one who speaks to you as you make the vessel that gets discarded at the start of Deltarune, and when you lose to the Knight it's believed that he's the one typing that th speaks to . and Toby says he originally thought up the idea for Deltarune before Undertale. In Entry 17 he talks about darker yet darker", and in Deltarune you enter dark fountains.Two of the fun value events, clam girl and Ice-ees, are directly tied to Undertale, in a way more obvious way then they're tied to Gaster. Entry number 17, which is basically confirmed to have been created by Gaster, reads: "THE DARKNESS KEEPS GROWING. THE SHADOWS CUTTING DEEPER. PHOTON READINGS NEGATIVE" In Deltarune, "The Roaring describes an apocalyptic event where, if too many Dark Fountains exist at once, the world will fall into chaos". It makes sense that he'd want to bring about the end of the world, he's like- literally associated with the devil (number 666). My guess is that Friend made a deal with Gaster, which is why he the same way he approached Jevil and Spamton.

Friend always helps a little when he's hurting you, cause he's a demon you can make a deal with In chapter 2 he neither hurts nor helps- just watches. In chapter 3 he sells a smile in the vending machine outside the sword room. The smile isn't actually free but you still recieve it? I think? In mic minigame he can either heal or hurt your health- though his damage takes off more points than his healing adds When attacking you in sword route in chapter 3 he drops candy that heals you when defeated. In chapter 5 when he harms you on the platforms you immediately fall to an area with an item that allows you to heal?

Gaster's entry 17 is about something different, not the expiriments in the True lab. That's why its voice recorded when literally all the others are typed. That's why there's a seperate entry 17, written by Alphys which can be accessed in the text files, which fits into the true lab narrative much better. Gaster had a seperate expiriment going on. When releasing Deltarune he calls it very, very interesting

Gaster couldn't have been talking to Alphys in entry 17- the timeline doesn't line up (Gaster worked under Asgore and the queen, Alphys doesn't know who the queen even is cause she's too young). Was friend one of the people he's talking to?

Gaster fell into his creation = Deltarune? (this theory falls apart beacause wasn't he scattered across time and space?) Friend helped him make Deltarune, with the cost that he was forgoten by everyone- including his loved ones, Sans and Papyrus? (theory also falls apart here because, like Alphys, Papyrus is too young to know the Queen cause he's surprised she exists at the end of undertale?)

What's the reason behind so clearly making Gaster a skeleton, if he's too old for Papyrus to have known about the queen, and by extension, him? How long do Skeletons live in Undertale? Is it long enough for Gaster to have known Papyrus when he was a baby, and Sans when he was a young child?

If Gaster's the one who created Deltarune (which is heavily implied to be the case), and he's the one coding all the meta screens, which are different in the code, then he's also the one who named friend in the code. He adds him to the code and just calls him friend.

Kris practices the occult with Catty, who is a cat.

Friend is the devil, and Gaster made a deal with him. This hinges on the line in Undertale's code about making a deal with a demon (demon a - demon d text), which Friend in the vending machine then refrences in chapter 3. Also, you find that line of dialouge about making a deal with a demon inside the code, and it can't be accessed anywhere else. A LOT of things about Gaster in undertale are found via searching or manipulating the code. This would make sense as to why Gaster added Friend, who is associated with hell and in Undertale might be the demon, into the game under the name "friend".

Gaster creates Deltarune/is the programmer for Deltarune, and then falls into the game? I'm basing this off the fact that river person says: beware the man who came from the

Friend is trying to find the knight. Or Friend made the knight?

Tokyo

Tokyo is the capital of Japan.