Sister.

Upon seeing the closeness between these two—how they care about each other—some fans prefer seeing Allen and Lenalee as brother and sister to “lovers”.

I think it isn’t happening for no reason. As we know, Lenalee thinks of her comrades as her family, living under the same roof called “The Black Order”. I see that this concept somehow affects Allen. The Order gives him his second family ever since Mana’s death. Of course, by the word “comrades”, it means that Allen is a part of Lenalee’s family, and in Allen’s case, Lenalee is a member of his new family.

People tend to see Allen as the little brother, while Lenalee is the older sister. This is because the official character profile in the manga mentions that Lenalee is 16, while Allen is “around 15”. Allen’s real age is unknown, but yes, that is probably true. Maybe he’s younger than Lenalee by a few months—we can’t be really sure because we don’t even get to know Allen’s real birthday.

It is said that a woman is usually more mature than a man of the same age. This means that, Lenalee should be a lot more mature than Allen. Both of them had a rough past, but it leads them to develop their character to be more mature. Who is more mature between these two?

Allen accepts the curse and moves forward, and Lenalee finally accepts her destiny as a Conformer. I think both of them can also be emotional in some ocassions. Lenalee appears to be naive sometimes, just like when she fell asleep in Allen’s room. Allen, on the other hand, often appears to be calm and mature but he often acts like a child too, especially while dealing with Kanda or Lavi. Lenalee is usually the one who tells them to stop.

Even in chapter 105, Allen admitted that he sees Lenalee as his older sister. In this chapter, Allen and his teammates heavyheartedly took the option to leave Krory fighting with the Noah twins, Jasdero and Devitt. Lenalee, however, didn’t want to do this and insisted that they should be together, because Krory was injured badly. Allen, seeing that Lenalee’s desperation as she was about to cry, shouted to her. Lenalee was surprised. He brought her face closer, and cupped her cheeks.

“It’s okay... This isn’t the strong Lenalee I know. You’re like a big sister, right? Lenalee.”

The words Allen chose was quite unclear, but I think he was trying to say that Lenalee’s rushed and emotional actions was different than her usual self. The Lenalee he knew is Lenalee who’s like an older sister to him—strong, calm, mature, smart, aware. Remember the Rewinding town case, where Allen and Lenalee were involved in an argument? There, Lenalee slapped Allen because he was trying to do something fruitless? He desperately tried to save the akuma, but he couldn't make it and it would risk his life. However, this time Lenalee was the one being reckless. The strong Lenalee would see the best choice for them and take the path—that was why Allen said that she wasn't being her usual self. She lost her usual strength, probably because she was feeling useless; she couldn't use her Innocence.

This scene is probably the only canonical claim of Allen about what Lenalee meant to him, other than the obvious: comrades. Maybe that is also the reason why fans like to see them as a little brother and his big sister.