Cloud and Aeris are from the Playstation game Final Fantasy VII.  He was a troubled mercenary and ex-member of SOLDIER, and she was a sweet flower merchant and the only surviving Ancient.  After they met, she helped him realize what true friendship was and he helped her carry out her dreams.
Why Cloud & Aeris?

It has been many years since Final Fantasy VII was released, and the one debate that still continues to this day is 'Who does Cloud really love?'.  Well, everyone is entitled to their own opinion, and there is no real right answer.  In our opinion, Cloud really loved Aeris, but that's not to say that he didn't ever care for Tifa.  I think he had a childhood crush on Tifa, and they became friends throughout the game since as we saw in the Lifestream sequence, Tifa and Cloud admitted they never even really knew each other that well as children.

Cloud had some huge gaps in his memories and personality at the beginning of the game, and in every scene with anyone up until he met Aeris he was pretty cold to everyone.  He didn't smile or laugh or joke around.  When he met Aeris, he started to let more of his true self out and seemed to be more comfortable and considerate.  She made him laugh and he was able to care about something other than himself.  When she was killed, he was devastated and actually cried, something he had never done in the game up to or after that point.  It was moving to see him show some real emotion, something that it seemed Aeris had awoken within him.  A common argument was that Aeris only liked the "Zack" in Cloud.  I don't believe that is true, because she openly admitted that although he reminded her of Zack and that was what she was initially attracted to, she knew he was different and wanted to get to know the "real" Cloud.

Cloud and Aeris are two of my favorite video game characters.  I just love their story and enjoy watching it continue to play out through Kingdom Hearts, Advent Children, Crisis Core, and anything else they decide to come out with.  The whole debate over who Cloud loved really doesn't matter to me anymore, and there is really no point in arguing it.  Just let everyone like the pairing that they want and then everyone is happy.

- Liz