Sunday is a Friday Monkey, and Animal Partner to Foster. It is very brave, and willing to fight with Foster, despite knowing it is weaker then Foster, very unlike most other Friday Monkey's.
It heavily resembles a normal Friday Monkey, but bigger and heavier.
Unlike normal Friday Monkey's, Sunday is brave, and willing to fight to defend his people. It was good enough to impress Foster, into making him his animal partner.
Sunday is stronger then the normal Friday Monkey, but was weaker then Foster, losing in one punch.
- Sunday is named that, because he was made on a Sunday, in-universe and in the real world.