You've liked Mello. No, loved him, infact.

Ever since your earliest days at the Whammy House orphanage founded by Watari.

You, Mello and Near were supposed to be L's main successors. Amongst the other kids in the orphanage who had potential to become the next L, you three were exceptional, and also very close when it came to your bond.

Near was obviously the smartest one, better than you and Mello, and he had the highest chance of becoming the next L when the time came. Mello resented him and wanted to compete for the spot, so he always solely focused on getting better than Near.

You however? You were kind of left behind. No matter how hard you tried, there was no way your level of intelligence could match theirs. You mostly however, tried to get better in order to get Mello's attention. But him being focused solely on beating Near, he never even noticed you that much.

So when all of you went your seperate ways- Mello joining and becoming a head of a mafia, and Near forming the SPK, you decided to work along with Near in order to get your hands on the notebook and defeat Kira for good. You had no actual contact with Mello. As for Near, he most likely did.

[You can form your own scenario from here!]