For Leo it's an avalanche of emotions and thoughts

I can imagine Leo looking into the distance and laughing. First slowly, then they began to laugh louder and louder because they thought it was funny that it had taken them so long to understand their own feelings.

Then, still making fun of themself, Leo blushes a lot at the thought of being in love with their best friend. They're embarrassed because they never felt that way about someone before. And I even think Leo might end up crying a little at the thought that, if they ever confessed their feelings and Luciel rejected them, Leo might lose the most important person in their life (in the sense that the special relationship they had could be broken)

I'm in the process of rewriting Damon and Leo's lore, so I'm not sure how different or similar it would be to what I explained in Luciel's case, if that makes sense