Angel and Junior's relationship isn't very deepened in the comics or cartoon. Their only appearance together was in "Para cada anjo, um pestinha!", but they haven't a good relationship: Junior seemed to like Angel, but Angel is clearly irritated with Junior, largely because of Junior.
At first, Junior did not seem to understand what Angel was, thinking what he was, from a character his father created to make him eat for an imaginary friend and even a superhero, until, after much patience from Angel, Junior realized that Angel was, well, an angel.
Angel and Junior don't have much in common, but the episode with them interacting sparked the fandom to make them friends, but nothing more than it, after all Junior is only 4. They could be friends because, even though Angel was angry with Junior - even talking to a little demon that he couldn't take Junior - , his angelic nature would allow him to forgive Junior easily since the other kids at Monica's gang have treated him worse than Junior and he is still their friend. On the Junior's side, he already seemed interested in Angel right away and if he stays in the same way that his character was developed in the comic books, Junior would probably try to get involved with Angel again if they met again.
There are controversies on the part of the Monica and Friends fandom who do not understand that it is a friendship ship and not a slash ship. Who does understand it, has no problem to ship them, but who doesn't, points out that it is a problematic ship that romanticizes "wrong feelings" because Junior be 4, as previously said. Anyway, there is no fanfic about the ship on AO3, mainly because the site is not very well known in Brazil, but also because this ship is mistaken for "wrong feelings" so the fandom is a little shy about mass producing content about.
- "Andudu" is named because of Angel and Junior's Brazilian names: Anjinho and Dudu, respectively. It may be translated as Anjunior.