[Aerith thinks. She has to be careful about how she says it. Somnus is probably a prideful person and she doesn't want to damage that outright. She also doesn't want to offend if she can help it.]

I was thinking that it can be a reminder to you that even in moments when you feel like you're alone, you aren't. Because you have me here in one way or another. [The plant, a reminder of her presence in Camelot, it seems.] I don't really think you're the type to rely on someone, but if there comes a time when you need help, I promise that you can come to me and I will try to help if you want it. Or I will simply listen. Whatever you might need from me, I'll try to give.

[She doesn't say it, but there's caveats to that. Aerith can't fix everything. And she can't give everything. At least, not without some regret attached, but she doubts very much he'd ever ask anything of her like that.]

So a promise that even if you feel you have nothing, you have me. And maybe... one day I'll be able to come to you if I feel like I have nothing, too.