Arnold first appeared in the episode "I spy with my little eyes" Where he was moving to Lola's flat next door. At first Lola thought that a family of wolves were moving next door causing her to be scared. Then they saw Arnold who's last name is wolf and he and Lola became friends.
And in "I'm extremely and absolutely boiling" he stops being friends with Lola, after Arnold won't share his ice cream with Lola when she drops hers, but then Arnold and Lola are able to build their friendship back up.
His personality is neutral.