The basics of Tifa and Reno's relationship are easy to decipher. During the game, Reno somewhat tortures AVALANCHE, so it's likely that Tifa harbors anger and resentment for Reno. Reno is also responsible for the destruction of her home, bar and the death of her friends. Holding him accountable for such actions, Tifa may not think too kindly on the idea of befriending Reno.

However, during Advent Children, the relationship might have a chance to shift. The two can be in the same room together without any negative fallout. They seem to hold no true resentment for things past, and it's suggested that they could move on as allies in a light extent.

Friendship is something that is hardly hinted at. During Advent Children their interactions are much more harmonious than their previous relationship in Final Fantasy VII. To venture into the land of friendship is something only fans can do. There are many fanfics depicting a harmonious friendship developed between the pair. I have no doubt that this is possible, as Tifa is kind and forgiving. On the other hand, her trust is not easily placed, so it may take time for such a relationship to develop.

Due to Reno's habits of being playful, he could prove to be an entertaining friend. I think that their personalities could compliment in that sense. However, overcoming the barrier that is the past might prove to be difficult. As far as friendship is concerned, they might have a ways to go.

Love is not mentioned in either the game or movie. A relationship involving these two is a creation of fanfiction authors, but it is very interesting. It is possible for the two to overcome the issues in the past and build a friendship. Therefore, it is also viable that they could grow in that friendship and eventually grow to love one another. If Tifa were able to put feelings aside for Cloud, for whatever circumstances, Reno might prove to be a perfect distraction

Despite being a slightly mismatched couple, they are really quite compatible. His playful attitude coincides with her light heartedness and they would never have a dull moment. This is evident through fanfictions that remain in character. It is wise to read some if you are curious about all this relationship craziness.