To cooperate.

Since he joined the Black Order, Allen had been assigned to several missions with Lenalee and the others. This allowed them to get to know each other, to cooperate. In fact, they made a good combo of attack.

These are the list of missions where Allen worked together with Lenalee.

  1. Rewinding town
  2. Finding General Cross (China)
    1. (separated) Lenalee: train, Allen: “vampire castle”
    2. (reunited) train station
    3. (separated) Lenalee: Edo, Allen: China
    4. (reunited) Edo
    5. Noah’s Ark
  3. Black Order infiltration

If you look at it, there might not be a lot of missions, but some of them were long-term missions, which stick them together for quite a while. Those times were sufficient enough to make them understand each other, appreciated one another, and made their relationship develop from just strangers to best friends. 

Well, they started to work as a team in Allen’s second mission. It was the first mission assigned to Allen together with Lenalee—a strange case. They came to a town where it seemed like the time kept on rewinding. Nobody could get into the town either. Komui's theory was, if the cause of this mistery is an Innocence, then probably exorcists can get inside—and it was true. However, they couldn't step out of the town before the mystery is solved.

When Allen and Lenalee splitted up to do some research in the town, Allen encountered an akuma attacking a clumsy lady named Miranda Lotto. Apparently, Miranda is the only one who noticed the anomalies going on in the town.

When Allen was injured because of the akumas’ attacks, Lenalee was the one who took care of his wounds in Miranda's place. Later, Allen and Lenalee concluded that Miranda is a Conformer, and her clock is the Innocence—which reacted to Miranda's wish that “tomorrow” would never come. After that, they came up with a plan to make Miranda get a job. They thought that if Miranda could get a job and felt more positive, the abnormality might come to an end.

In this mission, they had their first encounter with Noah family. Road Camelot was the girl who commanded those akumas to fight them.

Development

Chapter 153: Allen-Lenalee attack combination

Allen-Lenalee attack coordination had improved a lot compared than the Rewinding town saga. In this part of the story, Allen hadn’t known much about Lenalee—and neither had she. But after they had been engaged several battlefields in various missions, Allen and Lenalee had pretty much knew the other’s combat style.

Hence the Allen-Lenalee combination while fighting the newborn level 4 akuma. Lenalee had just recovered her Innocence—but the akuma already dealt great damage with their combo attacks. There, Allen stuck the akuma to the ground with his sword, and he knew that Lenalee was coming from the sky to adjust more power and gravity into Allen’s sword, causing more damage to the akuma.