πΈ Surprisingly generous with nicknames. He was a bit embarrassed to call the farmer anything but their name at first, but that went away pretty fast. The nicknames just slip out naturally, and eventually he rarely calls you by your name. Lots of 'kiddo' and 'kid' for older regressors, and 'baby', 'sweetheart', 'buddy' for younger regressors.
πΈ Big fan of rainy day activities, both indoors and outdoors. The moment he catches the farmer even glancing outside, he asks if you want to go for a puddle walk in the rain. He'll take you out with all your rain gear if you want, but he's also very willing to take you out barefoot in your clothes to splash around and get muddy. (Abigale always frets that you'll catch a cold, but Sebastian always loudly reminds her that it's an old wive's tale on his way out the door)
πΈ Frog! Catching! (and releasing, of course). Always brings a bucket on puddle walks to catch any frogs you might stumble across. Lots of reminders to be gentle with the frogs when you hold them, and you both wave them 'bye-bye' when it's time to go home.
πΈ During DND sessions with Abigale and Sam, if you're regressed you get to sit behind the DM screen with him and role his dice for damage and events.
πΈ Sometimes worries he's messing something up or doing a bad job as CG if something goes even mildly wrong. Knee scrape? His fault. Catch a case of the sniffles? Oh god, what if Abigale was right? Ahhh panic panic panic
πΈ Not super eloquent with comforting words, but pretty good at snuggling. Silently opens his arms to ask if you want a hug, or buried in blankets beside him on the couch while the both of you or just he plays his switch.
πΈ Likes to brush hair! He finds it relaxing.
πΈ Between Sebastian, Abigale and Sam, Sebastian is considered to be 'mom'. He's the most attentive and the most likely to keep the farmer out of trouble. If Sam or Abigale are looking after the farmer, they'll ask things like "Do you want to go find mom/mama?", jokingly referring to Sebastian.
πΈ Despite the mother hen jokes, he's very much an older-brother CG. When he and Maru were younger and got along better he really liked being a big brother, and it's nice to re-live that a little. He misses it.