Cristina seems to be an acquaintance of Lincoln, and attends the same school and formerly the same class as him. She has not had any speaking roles yet.
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 Lucy's bust of Edwin, and pretended that they were a couple, speaking romantically to it, 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.
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 comments about it (only getting angry at Luna for her meddling), and Cristina just disappears from the scene later.
She makes a cameo at a Dairyland queue in "Kick the Bucket List".
Since more recent episodes show Lincoln reciprocating Ronnie Anne's feelings for him, and he was also shown having a crush on Paige in "L is for Love", it's likely that he got over his crush on Cristina.
Cristina wears a navy blue blouse with a matching colored skirt, white socks worn high, blue shoes, and an orange headband. She has very curly ginger hair, and wears orange earrings.