FIRST PIC -> Their official relationship doesn't start until after "Tales of the Yokai" Episode. Before that, during Season 3, they are totally into each other but they wanna keep it low for now... then, in "Battle for New York" episode, Selina and Leonardo became more obvious about their attraction but again, they weren't sure if it was the time or not (but even Raphael noticed and he was like "Man, you want her or not, stop hiding it."
Season 4 Space Arc, the Fugitoid already knew Selina was special but he thought everyone knew it already so he didn't mention it (idiot). Selina's birthday came in mid of their space adventures and Leonardo was SO READY, he knew EXACTLY what to do so he wanted to surprise her to a trip to the moon! They discovered that Selina's scales and markings (even eyes) started to glow once she stepped on it... which shocked them both but Leonardo was never been more happy. Leonardo was about to propose marriage to her (remember in my AU they're young adults around 20') as a birthday gift, but- Lord Dregg interrupted and kidnapped Selina to use her lunar powers as a battery cell for his own planet. Imagine Leo's fury after that.

SECOND PIC -> They were totally lovey-dovey couple (while drama happens in the background) but... yeah. I'm sorry.

THIRD PIC -> Well, what else... they mentioned 90' but I did the 87' timeline instead because I thought they were funny.
Yes, 87' Selina appears in the crossover episode of Season 4. Leolina there is... totally lovey-dovey too but kind of goofy, he gifts her many things despite being enemies, but as the series are goofy af Shredder is like "She's supposed to be on my side, not their side, I'm gonna gift her stuff too to win her to be more on my side." And shit like that XD

FOURTH PIC -> Turning back to seriousness of what's going on with 2007 gen of Leolina, Selina of course is still in the Foot Clan but as Shredder died she was just with Karai working for Max Winters and the Stone Generals to capture the monsters across NYC. She was so happy to find out Leo was back from his trip, yes, they hang out when they can in secret (and Raph found out an excuse to be more mad at him that he hides secrets from his brothers too).
But as soon as Raph vs Leo ended and Leo got ambushed by the Stone Generals and the Foot Clan... Selina betrayed him. She was threatened to be the last fake monster if she didn't collaborate to capture one of the turtles to replace her and she had no choice but to betray him.