To talk.

While trying to help Miranda to get a job in the Rewinding town, Allen and Miranda worked in a circus. He worked as a clown, while she played the witch. During the break, Lenalee came to meet Allen and they talked.

Lenalee was amazed by how skillfull Allen was, and wondered how he could do that. Allen then told her about his past—that his foster parent and he were travelling performers, so he had to learn some acts to earn a living. In exchange, Allen asked Lenalee when did she join the Black Order. Lenalee then told him about her rough past.

That was the first time they began to get to know each other. Talking about one’s unpleasant past means that he trusts the person he’s talking to—this indicates that Allen and Lenalee had started to open up to each other.

Development

Allen Walker was very much the ladies’ gentleman—charming, and spoke in a really good manner. He was very polite in the beginning of the manga—you can tell from how he ended his speech with “desu” in Japanese. However, in the later chapters, after he got to mix with the Black Order members a lot, he talked in a less formal kind of speech. For example, he used “da” instead of “desu”.

Allen’s change of speech indicated that the relationship between him and his comrades—including Lenalee—had grown closer. They had became close friends. When your close friends are around, you feel like you could talk about anything with them—no formality included!