"She's one sick ass B-ball player." - Florida Man.
Waifu is the theorized girlfriend of Florida Man.
One explanation for Waifu's true identity is that she is the girlfriend of Florida Man. This is supported by the fact that his arm is explicitly around her in the original image.
Another argues that they are NOT boyfriend and girlfriend, but are instead in a platonic relationship. However, this theory is less credible because there is little to no evidence to support this claim.