Hi Annka (sorry if I wrote the name incorrectly), but you can find more facts about Toothache or about the Sign (your choice, although you still wanted more about toothache and why he/she/it looks like that).
Toothache is extremely, EXTREMELY anti-social, and refuses to make talk with ANYBODY at all. Even his interactions with the train staff are as minimal as possible.
Yet despite this, he has a soft spot for children (or anybody who acts like one). Of course, this goes out the window if said child acts like a total shitass.
It's this reason he's grown fond of Breathette, and has actually agreed to act as her caretaker whenever her parents call upon him. He's quite protective of her, and actually tends to worry for her, be it because of being afraid of her parents' repercussions or simply for her own safety.
And on the other hand, he HATES Madmask. So much so that Toothache WILL hurt the guy on sight.
Toothache (and his race in general) can emit loud shrieks that can deal genuine damage to both beings and objects.
BONUS: His name is Toothache because of his old design that WAS more tooth-centric. However, the name stuck after his redesign.