Iphone iMessaging and Data Usage?

If I imessage a friend of mine who also has imessage but I have Wi-Fi and she doesn't, does that use my data? Or vice versa. But if we both are using Wi-Fi then it's definitely free right?

If you're friend doesn't have a Wi-Fi connection, then it is considered a text rather than the usage of iMesssage. So it doesn't use Data it used Texts, hope this helps x