chillest guy in town

All the first Pokémon in everyone's teams are the ones I believe they owned first, chronologically, for whatever reason.

Constance:

Received Tepig when she began her journey through a professor who was a family friend, and always loved Constance.

Bidoof was an HM slave she felt too bad for and kept.

Appletun probably hung around the Café a lot.

According to its Pokédex entry, Absol is known to bring about disaster and bad luck, often shunned for this reason. However, it took a liking to Constance, since she didn't believe any of that BS and loves it anyway.

Tinkaton just felt right. I think Constance would wield a hammer if she could.

Herdier was the Blackwood family pet who refused to leave Constance's side when she was about to embark on her journey.

I almost gave her a Minun, and Ocean a Plusle.

Misha:

Kangaskhan was his mother's before she passed. It exclusively listens to him now.

Loudred probably has some "sick" amplifying capabilities and loves blasting Misha's awful rap.

Misha found an abandoned Braviary nest, and rescued the egg containing Rufflet.

Amoonguss probably supplies some...pleasant recreational spores.

Encountered Politoed on accident while fishing, never let him go.

Simisear just felt right; "hot-headed."

Noel:

A Roselia appeared in his mother's garden one day when Noel was a kid. They were obsessed with each other ever since.

Drizzile was the more "traditional" starter, gifted to Noel when he embarked on his journey. They often wear the same expression.

I think some neighbor or someone didn't want their Purrloin anymore, so Noel took it in.

The rest just felt right.

Ocean:

Parents are completely against owning Pokémon as they think it's "inhumane." However, Chatot followed her on the way home from school one day, and simply didn't leave her alone - and Ocean didn't want it to. Chatot is incredibly loud, but manages to be completely silent when it's at home, keeping its existence a secret from Ocean's parents. It helps during Choir rehearsal.

Emolga is known as one of the most annoying Pokémon to deal with in the game. Self-explanatory.

Servine was the typical graduation/journey-starting gift. Doesn't always listen to what Ocean says, but is there when it counts.

Ocean caught Glameow because it was cute, not knowing it evolved into Purugly. (I thought it was also a fun metaphor for Ocean's sweet facade/ugly inside, in canon.)

Lapras saved Ocean from drowning once (because ironically, she can't swim) and never left her side.

Ocean enjoys going on night flies on Noctowl's back when she gets overwhelmed. (Also owl association with being smart?)

Penny:

Zorua was in the family from her birth parents, probably a symptom of some kind of illegal breeding operation. It only ever really loved Penny though. When she went to foster care, Zorua was originally supposed to stay with her parents, but tracked her all the way there and never left.

Togekiss is known for being lucky.

Ditto I thought was fun because it takes on the face of others; is largely faceless. Same for Mimikyu.

Froslass's Pokédex entry says it's the possessed spirit of a woman who died on a freezing mountain. Penny would totally be into that.

Same trauma with the Cubone/Marowak and identity/family.

Ricky:

Cats. Obviously

Reuniclus looked like some kind of Zolarian extraterrestrial thing.

I think Ricky enjoys tinkering and creating stuff like new Pokéballs, battle items, etc. Klinklang would help.

Meowstic is very empathetic, and taps Ricky furiously on the leg when it senses him getting stressed out.