HMMM. [potential game spoilers ahead]
dirk plays pacifist and neutral, gets partway through geno and becomes too upset to continue. stresses himself out thinking he should finish it, but can’t because it makes him feel too bad about himself. he would accidentally kill toriel but not reset because he would be committed to following his choices through, i think. favorite character… i think he would lowkey really like alphys.
jake would do neutral to pacifist, definitely not geno. i could maybe see him resetting to save toriel if he thought of it. he would do a neutral route the first time and get some LV i think. not in the ruins because the toriel telling you not to guilt is real but later on, when things get more frustrating and you keep dying. his favorite character is undyne. OBVS undyne. but he would like all of them, especially papyrus and alphys i think. flowey might be a bit close to home for both him and dirk, remind them a bit of the AR.
roxy plays all the routes and does excellently. defeats geno sans with unfair ease. probably does one of those really irritating no-hit runs where she gets thru geno without ever taking damage. people are slightly horrified by her laughing at parts of geno but she doesn’t give a shit. her favorites are the temmies. :3 but i think she would like undyne a lot too tbh.
janey… jane would play neutral, then pacifist, and then be done, i think. unless she was pissed when she was playing, but i don’t think she has the patience for geno. i could see her doing a high LV neutral route out of frustration and then feeling a bit guilty about it after. her favorite would be toriel and she’s definitely rooting for toriel and asgore not to get back together.
dave plays the pacifist run where you dont have to reset fully after photoshop flowey on account of never having killed anyone, because he heard that you can do a route like that and chose to not be attacking if he didn’t have to. would figure out the dodging + saying fun things bit pretty quick and have too much fun with it. his favorites are all the most hilarious random encounter monsters, sans, and nabstablook. and he races the snails too many times.
rose plays neutral, then geno, then pacifist, just to be contrary. betrays herself by thinking muffet’s fangs are hot. actually disliked geno a lot but played it so she can talk loftily about its implications and dirk can’t do shit if he wants to debate with her because he flunked out of it. maybe her favorite is chara for the moral ambiguity and being influenced by the player’s decisions. (i generally subscribe to chara as the narrator theory.) also flowey. she would like flowey.
karkat plays with the temmie armor. lmao. he complains the whole way about encounters being too hard but stays pacifist. he virulently complains about all the characters, calls toriel the only sensible one, and secretly likes mettaton.
i dont know if i have enough solid opinions on the other characters… jade would love that you can throw the stick for the dogs. she and jake both pet lesser dog until the depths of the universe are plundered. terezi might like asriel, but her favorite part would the the color tile puzzle. if john tried geno he’d fail out when you have to kill the dogs. callie probably just watches others play with mild bemusement, and her favorite is alphys.