Ryelsi

Ryan Evans and Kelsi Nielsen.

HSM
In the first movie, Ryan shows indifference to Kelsi, but they sit at the same table in the cafeteria (as shown just before Stick to the Status Quo), and Kelsi and Ryan share a look while Sharpay oversees the cafeteria, causing some fans to already see a future couple in them.

HSM 2
In "High School Musical 2", however, Kelsi is shown to have a locker next to Ryan's, and they presumably spent a lot of time together planning how to get Troy and Gabriella back together. In addition, Ryan and Kelsi actually speak to each other during the movies, and it is implied that Ryan supported Kelsi's ideas for the talent show before agreeing to direct the Wildcats ("Oh, hey. Kelsi's got some great new ideas to spice up the talent show..."). They are also the only characters to wear hats regularly, and their hats often match.

HSM3
Ryelsi is the pairing in "High School Musical 3", as Kelsi is Ryan's prom date. They sing a duet together (along with Troy and Gabriella) called "Just Want To Be With You", after Ryan is ordered by Sharpay to take Kelsi to prom in order to steal "Just Want To Be With You". Some fans have interpreted this change in pairings to ease parent's fears over a character in the series, pointing out that Ryan did not have a girlfriend until the very end of High School Musical 2.