Komui had two greatest fears in the whole world. The first fear was the pile of paper work that he had been forced to sign by the evil Rever. The second fear he had was basically any harm that had been "dealt" to Lenalee. The idea of innocence, fighting, her falling in love scared him greatly. Her heart had weighed more then anything in his life and he never wanted her heart to break by the hands of a man so he tried his best to keep her away from men.
It had seemed to be working for a while until instead of her falling for a man, men started falling for her. She was beautiful, smart, strong willed, and compassionate, which some men had tried to take advantage of. Of course, Komui had developed ways to stop these men from their perverted attempts by creating many machines, such as the Komurin series. It was not until the one, Allen Walker, had come into the Order when he noticed a strong change in his sister. She had started to talk about him, started to be around him, started to even let her guard down around him.
The day he found that Allen had actually kissed his sister was the greatest shock to him. He vowed blood, he vowed gore, he vowed the death of Allen Walker, but then it struck him, the boy was never the type to break the hearts of his loved ones, he was never the type to betray his friends, he was never the type to harm anyone if there was an alternate way of solving a problem. So when he had found out that Lenalee had fallen in love with Allen Walker, he had given his permission, knowing Allen Walker could protect her in everyway possible and love her greater then any man in the world, even greater then he could.
I know it's short but school and tennis like to disrupt me from my writing creativity. I had this in mind for a while actually and I've been trying to think a little about how he might feel about Lenalee and Allen. I'll try to update more once I catch up with the Anime itself because I'm only current on the Manga. R&R :]. Makes me happy.