To grow.

To see how good or bad a relationship is, you can try evaluating how your partner influences you. Does your friendship with a certain someone give you bad influences? Or does it help you “grow”?

In Allen and Lenalee’s friendship, i think they are giving good influences to each other. These influences were getting visible after their separation in China.

The boy

Allen Walker’s raison d’etre, at first, was to fight the akuma, and live for akuma. The only thing he treasured was akuma—because of his sin for turning Mana into akuma. However, after his Innocence got destroyed by Noah, and he was left in China to retrieve it, he finally realized that his world had grown.

Chapter 83: Allen realized how his world had grown

This is most likely an indirect effect of what Lenalee told him about her “world”, that every one of their comrades are a piece of the world. He was no longer alone—he had his new family in the Black Order—a place for him. At that point, he determined that he loved both akumas and humans—and that they would bring salvation to both of them. His determination brought him to all new and improved Innocence, Crowned Clown.

With this new Innocence, Allen exorcised the level 3 akuma. After some preparations, he boarded the Ark that akuma used, hoping to meet his teammates.

The girl

Chapter 110

In the past, she was a very problematic kid. She didn’t want to be “exorcists”. She wanted to escape from her destiny of being a conformer. After her brother joined the order, she started to accept her duty to be an exorcist. However, deep inside her heart, she was still in denial. She still hoped that everything was just a long dream—the Earl, akumas, Black Order. It just showed that her heart was still unstable. Afterall, she was just a little girl who was forced to carry the weight of the world in her shoulder—along with some other youngsters.

After seeing Allen’s attempts to protect their comrades—her world as she told Allen—Lenalee was aware of the truth she had to face. She noticed how Allen had grown during their separation, and she reminded herself that she had to work harder as well.