hi hello PokĆ©mon anon here (side note, if not taken, can i be āļø?)
i think i'll do one character at a time, because i know this will get long if i do more than that, and also so i have time to think about this
so i think shane would have (in dex order because that's easier):
metapod (according to the yellow dex, it "hardens its shell to protect itself", and the red/blue dex "is vulnerable to attack while its shell is soft, exposing its weak and tender body". shane has a tough metaphorical shell, and a very soft inside. feels fitting)
pikachu (crystal dex: "when it is angered, it immediately discharges the energy stored in the pouches in its cheeks". while shane may not have electricity stored up, he does seem to be the one to lash out when angered or annoyed)
oddish (no particular reason. apparently oddish can be mistaken for a clump of weeds so maybe he pulled it out and decided to take care of it because he felt bad)
psyduck (look... i love shane... but i do think he is tormented by chronic headaches, like psyduck.)
growlithe (red/blue dex "very protective of its territory. it will bark and bite to repel intruders from its space". shane does not like strangers getting in his space, feels fitting for him to have one)
galarian ponyta (originally intended for jas, it bonded to him by reading his heart [shield dex] and now he and jas share it. he won't say anything because jas loves it so much)
grimer (appears in filthy areas, born from sludge... seems fitting for him to have one as he works in joja mart)
shellder (red/blue dex: "its hard shell repels any kind of attack. it is vulnerable only when its shell is open", much like metapod, feels fitting for shane)
gastly (maybe it's the ghost of his former friend. maybe its just some random gastly that saw shane and latched onto him. who knows)
snorlax (listen... i love shane. however. i think that if he could, if he didn't have to work, he would just eat and sleep)
crobat (a friendship evo to show that he's good with PokƩmon and not always angry/rude!)
mareep (he lives on a farm, enough said)
miltank (again, lives on a farm)
torchic, combusken, and blaziken (HE LOVES HIS CHICKENS SO HE HAS ONE OF EACH. instead of a blue chicken he'd show us a shiny torchic)
shroomish (this one is simply based off of shroomish's expression. the perpetual frown.)
absol (shane seems to be the type of guy who thinks badly of himself, so it seems to make sense to me that he'd have a PokƩmon that is known as a doom-bringer)
wormadam - trash coat (legends arceus dex says that the coat is the perfect way to evade the detection of predators. i think shane just wants to not be perceived, so it fits)
riolu (according to the platinum dex "it has the peculiar power of being able to see emotions such as joy and rage in the form of waves", so i think it would make a good emotional support PokƩmon for shane)
joltik (i have no particular reason. i just think shane deserves a little guy. as a treat)
hawlucha (hawlucha seems to be a reference to luchadors, so for a sports fan, makes sense for him to have one. even if hawlucha are known for having flashy finishing moves and shane probably does not like having attention on him)
mudbray (again, on a farm)
passimian (passimian is potentially a reference to quarterbacks in american football, and we know shane is a sports fan, so it makes sense to me)
wooloo (you're probably getting sick of this reason, but again: he's on a farm)
clobbopus (shane is a sports fan, the marks on clobbopus' head seem to resemble wrestling masks, feels like a good fit)
snom (i like snom. i like shane. he gets a snom)
lechonk (i've lost track of how many times i've given the reason but: farm)
maushold - family of three (he has a small family when you meet him. it makes sense to me)
and that's all! sorry it got long lol. till next time!