To be cooped up – Stuck somewhere inside with not much room to move (think “chicken coop”)
Isolated – Alone and away from other people / society / civilisation…
Billy-no-mates – Casual term for someone with no friends (British)
Solitude – Being alone in a positive sense (eg. The beauty and calm of solitude)
To keep to oneself – To avoid other people and stay alone (neutral tone)
Recluse – Someone who doesn’t see other people and is always hidden away alone
Extroverted – Someone who gets energy from being around other people (often a confident and outgoing person)