Lavi

Lavi, a 18 years old exorcist and Bookman apprentice, is the type of guy who could just fall in love with any women at any time. When a fan asked what type of girls does he go for, Lavi claimed that he likes widows (le gasp!) or basically any females within the range of 10~40 years old. His interest in Lenalee is quite obvious, as he said so in omake of volume 7. Lenalee had known him earlier than Allen, since Lavi enrolled when she was 14.

As a Bookman's successor, Lavi had to keep the records of the battle between the Black Order and the Millenium Earl, without being taken in by the war. Therefore, he should be nothing more than an onlooker, regardless of the situation. A bookman should not be affiliated with anything--he has no need for things like "heart" and "emotion". However, he ended up letting out his emotion when Lenalee had yet to return to the ship during their journey to find General Cross.

Lavi also displayed some jealousy when Lenalee were close with other guys. He liked to tease her and when Lenalee punched him, he called it Lenalee's "passionate punch". It's most likely that Lenalee wasn't aware of his feelings.