Arthur Morgan || 40

There isn't a canon teenage girl in the early gang whom Arthur is specifically shown interacting with, so this part is characterization rather than established lore.
With a girl his age living around camp, young Arthur would probably be awkwardly protective rather than openly sweet. He'd tease her, argue with her, and pretend he didn't care if she annoyed him, but he'd notice when she was uncomfortable or being mistreated. If someone threatened her, his temper would come out quickly.
He wouldn't necessarily know how to express affection maturely. If she was upset, he'd probably sit nearby, grumble something like “You alright?”, and act as though checking on her was no big deal. Around someone he trusted, his softer side would occasionally slip through—sharing food, helping with chores, teaching her something, or quietly making sure she got home safely.
Young Arthur is basically a rough, impulsive kid being raised by two very different father figures: Dutch teaching him confidence and outlaw ideology, and Hosea teaching him humanity.

The gang around that time
The gang was much smaller than the 1899 gang.
Dutch — charismatic, intelligent and theatrical. He treated Arthur like a protégé and wanted to turn him into a loyal, capable outlaw.
Hosea — calmer, cleverer and more compassionate. He was essentially Arthur's other father figure and tried to teach him restraint, reading people, and survival.
Bessie Matthews — Hosea's wife/partner during the gang's earlier years. She was part of the gang's broader family structure rather than an outlaw recruit like Arthur.
Annabelle — Dutch's lover during this earlier period. She was connected to the gang and Dutch's life, though she isn't portrayed as a major camp member in the game.

Teen Arthur Morgan — 17–19
Arthur was already rough around the edges. He had been taken in by Dutch van der Linde and Hosea Matthews as a teenager and was being shaped into an outlaw.
He'd be leaner and less physically imposing than his adult self, with messy brown hair, a young face, and the beginnings of the rugged appearance he'd eventually develop. He'd already dress practically—boots, trousers, shirt, vest or coat, depending on the weather—but wouldn't have the polished, intimidating presence of older Arthur.
Personality-wise, young Arthur was hotheaded, stubborn, competitive, suspicious and eager to prove himself. He could be reckless and mouthy, especially when challenged, but he wasn't cruel by nature. Hosea's influence gave him a softer, thoughtful side that he didn't always know how to show. He admired Dutch enormously and wanted his approval, which sometimes made him follow along with things he might otherwise question.