It is unknown how long Lester has worked as a mascot in the park, but possibly a very long time.
He is very cheerful and positive, and is presumably very good with children (due to his friendly nature). However, he is persistent to get certain other people's attention, such as with Max when he turned away.
Lester Possum first appears in the movie, where he walks up to Max and says hello to him. Max ignores him (due to being so annoyed about his dad taking him to the park, and because of Lester thinking he's a kid), but Lester attempts to get his attention by asking him who's his favorite possum. Max then tells him not to touch him, and Lester asks him what's wrong. He starts to think that Max is sad and gives Max a big hug (much to his anger). He then tells Max that he feels all good and fine, but Max angrily slaps him in the face (flipping his costume head backwards) and tells him him to beat it. The dizzy Lester then staggers slowly away, and a group of children tackle him and drag him away. Lester isn't seen again for the rest of the movie, although it is possible he remains in the park.