Kimberly Hart was first introduced as a slightly ditzy girl who would do anything to help a friend.  Her closest friends were Jason, Trini, Zack, and Billy, all of whom she went to school with.  Kimberly excelled in gymnastics and was a bit self-centered, although always a good friend.  When the evil Rita Repusla was released from her imprisonment, the inter-dimensional being Zordon needed to form an elite fighting team to protect the planet from destruction.  He brought Kimberly and her friends to the Command Center to be that group - the Power Rangers.  Jason was the Red Ranger, Trini the Yellow, Zack the Black, Billy the Blue, and Kimberly was the Pink.  They were given special powers, such as the ability to "morph" into their more powerful state , and mighty machines known as Zords to help them fight against evil.  They had to keep their new powers a secret, and continue to live as normal lives as possible all while being the Power Rangers.

Tommy was introduced in the first part of a five episode arc known as "Green With Evil".  He was new to Angel Grove and Kimberly first saw him when he was competing in a martial arts tournament against Jason.  Their match ended in a tie, and Kimberly found herself attracted to the new guy in school.  Tommy and Kim first met when he stopped two guys at the school (Bulk and Skull) from picking on her, and they made plans to meet after school.  Tommy never made it though, because he was abducted by Rita and transformed into her Evil Green Ranger to fight against the Power Rangers.  Tommy proved to be a powerful opponent for the Rangers and finally, in the end of the "Green With Evil" arc, he was purged of his evil powers and joined the right side.  He became the sixth member of the Power Rangers as the Green Ranger, and brought with him his Zord and strong powers.

Throughout the series, Tommy and Kimberly became close friends.  They were always there for each other and could depend on the other no matter what.  Whenever Kim was in trouble, it was always Tommy who was there to help her out, and it was always Kim that Tommy turned to in times of distress.  Tommy and Kim shared their first kiss in the park when she agreed to be his date for an upcoming dance.  They eventually began dating and everything seemed to be going well.  Tommy's powers were lost in an episode and he left the show for a bit, but returned as the White Ranger, the most powerful one, the new leader of the group.  Everyone was thrilled to have him back, but none as happy as Kim.  There was a chain of episodes where Kim had her power coin stolen, and then another few where Kim was hurt after she fell off the balance beam, but Tommy remained by her side through everything.  Eventually, Kimberly decided to leave Angel Grove for Florida to train for the Pan Global Games at Tommy's urging to follow her dream.

One thing I really liked about "Power Rangers" was how the characters grew throughout the series, by way of character development.  Kim started out as a little conceited and flaky, but in her last episode she was an independent, caring young woman.  Tommy started out as a normal guy who was really interested in martial arts, but eventually came to be the well rounded leader of the Power Rangers and the best friend anyone could ask for.  Tommy and Kim went from awkwardly flirting to being in a comfortable relationship.  These were nice transformations to watch, and made the show's (many) cheesier moments bearable.  Tommy stuck around through the Zeo and Turbo series' and then returned again for Dino Thunder.  Kimberly only returned once, where she played a small role in the Turbo movie.

If we ignore the letter storyline, Kim and Tommy could have a very happy future.  They had a very realistic relationship, which was surprising for this type of show.  Unlike most dating couples on TV shows, they were never constantly hanging on each other or making out, and they had separate friends outside of their relationship.  They never jumped to conclusions about the other, and were never insanely jealous when someone else spoke to them.  I believe that Tommy and Kim could have stayed together through her absence and hooked up again when she returned, since they both seemed committed to the other.  They could have stayed with the Power Rangers, but maybe as more of a behind the scenes type of role rather than out there fighting, since the group seemed to rotate so much.  They could have gotten married and had children eventually, because (had the show not messed with their relationship) they were one of the few couples from a show like this that I could see staying together forever.

However, facts are facts, and the fact of the matter is that Tommy and Kim did not stay together throughout the show.  He even returned to the show in the "Dino Thunder" season to help the current group of Power Rangers.  I didn't watch that season, but I doubt Kim was mentioned or if Tommy was involved with anyone.  I guess it is always possible that they could have gotten back together after the last season he was in.

This quote was in the episode "Storybook Rangers", when Tommy and Kim were at their school's book sale.  Kim found one of her favorite fairy tale books from when she was little, and Tommy (ever the sweetheart) bought it for her as a present.

Kimberly: "I love fairy tales. I've always wanted to be a princess, and get rescued by a handsome prince on a white horse."

Tommy: "Well... how about a white tiger?"

Kimberly: "That would work."