Lenne’s lover is a man named Shuin who lived in the same time as her. Shuin was a star blitzball player of Zanarkand. Though it is not specified how they met, it is most likely that it was due to their social status that they probably encountered each other more than once.
It’s assumed that since Lenne is a summoner that Shuin was her Guardian. Since around the time they were alive, there was no need for a summoner to partake on a pilgrimage and no need for a Guardian to protect the summoner. Shuin certainly does act overly protective of Lenne, much to the point that he became obsessed in ensuring her survival during the Zanarkand-Bevelle war.
Taken from what little information that is offered in the sequel, one can tell that Shuin and Lenne really loved each other. Shuin is a man who seems to have darker and violent tendencies and not quite that sane. Lenne on the other hand is gentler and is Shuin’s voice of reason. She can instantly calm him with her voice and a simple touch can bring Shuin into a peaceful state of mind. This is how they compliment each other and without Lenne, we can see how a thousand years of separation brought about his mental degradation.
They live up to the meaning of being two halves of a whole. Shuin is certainly no pushover, in fact, he’s pretty strong and a clear warrior since he seems to know how to use the sword. While he is physically strong, he lacks in spiritual and mental strength and this is where Lenne completes the whole. Spiritually, she’s strong and gives Shuin the spiritual strength to continue on, even after the events in Bevelle, it was simply the thought of Lenne that drove him to survive a thousand years as a spirit. Though since Lenne is in need of someone to protect her physically, it gives Shuin another method of defining himself, he’s her lover, but also her protector. Her need for a protector allows Shuin to feel needed.
Shuin’s obsessive desire to protect Lenne was really what brought about their downfall and their separation after death. Shuin’s spirit wandered Spira searching for her while Lenne resided within the Singer sphere, though that doesn’t meant that their story ends there.