Theory - The Scientist Alphys Used to Work With is Napstablook

In one of the True Lab Entries (Entry 19), a line stood out to me. "i don’t even answer the phone anymore." Sans still uses his phone. Though we never get really his number, at the end of each neutral run he dials us. In the Stained Apron area in Hotland, we get a friend request from Napstablook that “...seems to have already rejected itself.” Napstablook may have an UnderNet account, but he definitely doesn’t use it.

Though Sans is the most famous character in the game that uses all lowercase (hence why people think he was the other scientist), when he’s serious he uses capital letters. The entries in the True Lab have an undeniably serious tone, yet the entries are still in lowercase. But there is another character that uses all lowercase - Napstablook.

One of the entries in lowercase (Entry 21) tells that the person who was writing in lowercase now spends all of their time in the garbage dump. “it’s my element.” could mean how Napstablook feels like garbage. Napstablook also saves us from Mad Dummy in the Garbage Dump and only lives a few rooms away from it. Entry 20 is about how Asgore found a cute teacup in the shape of the writer. Sans’ form is a little too complex for a teacup to accidentally look like, but Napstablook’s form is simple.

Napstablook helping Alphys create the Amalgamates (and Flowey) would explain why he’s separated himself from the rest of society and why he spends most of his time crying and being alone.

Mettaton and Napstablook have a close relationship, as seen with Mettaton’s diaries and the call at the end of the Mettaton EX fight. One of the lowercase entries is this: “now that mettaton's made it big, he never talks to me anymore. ... except to ask when i'm going to finish his body. but i'm afraid if i finish his body, he won't need me anymore... then we'll never be friends ever again. ... not to mention, every time i try to work on it, i just get really sweaty…” The bolded part is weird for Sans to say. Sans isn’t bothered about how many friends he has, but this is something fairly normal for Napstablook to say.

If Sans were the one writing the entries, he’d be writing in Comic Sans. But the lowercase entries aren’t written in Comic Sans. They're written in the font the whole game uses, except for special characters.

This is my theory, thanks for checking it out.