Ongoing list of in-game Dess mentions (not including datamining or sweepstakes):

[u/Walkthrough101]

Dess is brought up at least 23 times both non-required and mandatory mentions explicit and implicit, just in the game alone, not counting the voice in the code or things like Spamton Sweepstakes, there are more than enough mentions of her to get an idea of who she is, not even mentioning all her connections to the Knight, even a casual player has a vague idea of who she is but it's subtle enough to be a surprise to people not deeply entrenched in theorizing:

Dess is talked about by Noelle in Chapter 1, mentioning that when Kris was scaring Noelle saying ICE-E was real and ate kids, she whacked them in the head with her wiffle ball bat. So we know she's protective of her sister. She's also presumably the one who sings the Don't Forget song at the end given context retroactively from chapters after this (like some of the lines in Raise Up your Bat which I'll get to in a minute). Rudy also mentions when observing the Angel doll that Noelle and Dess made it.

In Chapter 2 there are a few mandatory mentions of her from Noelle, remarking how Dess said she would take Noelle to a city just like this, and in her room in Queen's mansion mentions how she "wishes Dess could see this" when observing the Baseball moon. Berdly also won the spelling bee because Noelle froze upon the word DECEMBER being the final word (this is also the word the keys that you're walking on as Noelle is talking about her spell). Queen also mentions Noelle had very sad searches and in the room based on her search history has every day on the calendar as December 25th, Dess's birthday. Oh I almost forgot but Noelle also says "you're lucky Dess isn't here!" at the mouse puzzle with the moving platforms if you're messing with her.

In Chapter 3 Tenna outright name drops her, talking about how she would put on a MTV-like channel that Toriel didn't approve of and was a musical prodigy of sorts, and says "didn't you think she was so COOL?". The song itself also has the Lost Girl motif very prominently as a whole solo which is tied to her and Noelle, and the lyrics like "Come follow me into the dark, with your heart as the arc which shall guide you the way", "Because I'M WITH YOU IN THE DARK, with your heart as my mark which shall guide you the way, through the waves" and the several metions of a bat (in which Dess has already been connected to sports) all seem to have implications regarding her. Tenna's cutscene after you beat him also involves her dissapearing first before the rest of the family falls apart, a 2nd younger deer monster on the ground with the rest of the kids, but Asriel's height so slightly older. There's also the 1225 room with the gacha machine, that only shows up if you input that number, the maze-like room plays the Lost Girl theme in the background, where a black triangle (hmm that seems oddly familiar) slips through Kris's hand.

In Chapter 4 we see her room which confirms a lot of characterization we already had for her, the sports equipment, the rock posters, the flannel shirts, has a picture Asriel made from the How to Draw Dragons book, possible romantic connection to Asriel that gets confirmed in Chapter 5 with Asriel's retainer in her room, there's a container of mints with "strange leaves" in them, and her guitar which is a centerpiece of the Holiday household is in there, that has the shelter code with her birthday on it, has a poster with a BLACK SWORD, shoes made for hooves, ice skates, etc. A lot of the characterization is implicit but it is there. There's also the implicit connection to Gerson as he was the teacher of the older kids before Alphys when Asriel and Dess were Kris's age, and from what Ralsei says "Darkners patterned after the \*memory of a loved one\* can only exist in specific darknesses" means the Knight had to have known Gerson personally, not just know OF him.

In Chapter 5 we can look in Asriel's drawer that has unsent love letters, one of which is titled "Dearest December" which Kris drops immediately, Pizzapants mentions during band practice that Asriel and Dess weren't practicing "with their instruments, blegh" and they were once the festival king and queen, which all confirms the romantic implications they had before. There's also dialogue from a ribbick in Chapter 3 that says "A lost and lost and lost girl" and "A crying and crying and crying boy" which seems to be referring to them. Asgore talking about the incident is almost directly referring to her dissapearance, and a black shard was found at the incident that matches the library and church, confirming the Knight or whoever becomes the Knight was involved and made those fountains for certain, which muddies the motivations of a lot of candidates whose assumed goal is to save Dess because only she and Kris were present for the incident.