You can always just search for their username again, start a conversation by sending a message, and go into settings to add that person.
Or some trouble will be saved if they contact you first.
Other than that, though, I don't think it's possible. If either person doesn't know each other's username, they won't be able to converse.
I wrote an email to Kik's support team claiming that something went wrong with my app and all my contacts were deleted. In reality I had deleted them myself, but I wanted them back.
It took them a while to get back to me, and I had to prove it was my profile, but they restored my contacts eventually.
I think they have them backed up somewhere. Anyway, go their site and look for the support email, or check for their official Twitter account and tweet them, There's also a help link in the Kik app.
In general, there is little chance to retrieve deleted kik messages, as Kik doesn't store data that has been deleted from their servers. Anyway, if you want to verify whether you still can find deleted kik contacts on your iPhone, you'd better find a tool on Google, in my experience, imyfone d-back is a good option, you don't need to purchase that tool if you only want to view iPhone data.
Didn't find the answer you were looking for?
