Cristina is one of Lincoln's schoolmates on The Loud House.
Cristina seems to be an acquaintance of Lincoln and goes to the same school as him. There hasn't been any vocal interaction between them, and she has not had any speaking roles yet. Lincoln has (or at least had) a crush on her, though the feeling does not appear to be mutual.
She first appears in "Making the Case". When Lincoln wanted to apologize to his sisters for posting his video of them displaying their most embarrassing secrets and moments by making an even more embarrassing video about himself, one of the things he did was put a photo of Cristina over one of Lucys dolls and pretend that they were a couple, speaking romantically and attempting to kiss it. When Cristina saw this, she was rather uncomfortable about it and, as Lincoln explains, switched classes presumably to avoid making any further contact with him. It is unknown if Lincoln still likes her after that.
In "The Green House", she makes a cameo at the beginning of the episode fawning over the picture of the polar bear alongside her classmates.
Cristina later makes a cameo in "For Bros About to Rock", where Luna notices her and thinks she's perfect for Lincoln, apparently not recognizing her from Lincoln's embarrassing video in "Making the Case". Lincoln doesn't make any comment about it and Cristina just disappears from the scene later.
Lincoln has likely since gotten over his crush on Cristina, since later episodes show him pursuing the attention of other girls (such as Paige in "L is for Love" and the new girl in "White Hare"), not to mention secretly reciprocating Ronnie Anne's feelings for him.