Angelina and Unknown Boy are romantically involved with each other. They have spent some time together and have been seen embracing at school, so are believed to currently be boyfriend and girlfriend. However, their official status - whether they're simply flirting and not exclusive, or in a committed relationship - has not been properly confirmed.
Lina and Unknown Boy first met at Brooke's party in Episode #15, It All Comes Crashing Down, when Unknown Boy told Lina she "really is the prettiest cat at this party". This leads us to assume they started the relationship at, or not long after the party.
In Episode #21, What's Done is Done, Lina and Unknown Boy are seen embracing in the cafeteria, a clue to the fact that they are currently together.