Just thinking about how excited Copper would be when Toriel brings Frisk home.
Because, he's not seen another human in, like, 5 years. Maybe even 6. Hell, he's barely spoken to ANYONE except for Toriel in all that time. The other monsters are alright but he still feels off with them, the dummy's not much for conversation and Flowey's... Flowey.
So when Toriel brings this new kid, this human kid, Copper almost immediately decides 'yep, we're friends'. Because he has no idea Frisk plans on getting out of the RUINS. To him, this is a new kid, a sibling and a friend to spend time with. He immediately wants to teach them everything he knows, what the best food is, the trillion snail facts, why it's called a water sausage.
Of course he'll guide them through the Underground. After all, it's his duty as a big brother, right?
(Wait until you find out I have the worst betrayal planned for the genocide route >:) )