Boy meets girl.

Quick and short, there was nothing spectacular or enthralling when she was walking past him. It was ordinary.

 

Truth to be told, Allen and Lenalee's first meeting is not my favourite scene of characters' encounter. There's no shoujo sparkles (obviously—I don't like those either), no warm and fuzzy bubbles... no falling leaves whatsoever. It all happened quick and short, and maybe it even gave out a not-very-friendly impression. They were just two different individuals, stranger to each other, being in the same place at the same time.

So how exactly did it happen?

the boy

So, after a long journey, Allen Walker found the headquarters of Dark Religious Organization (will be referred as Black Order in later parts) which his master, Cross Marian, talked about. He went there in order to become an official exorcist.

the girl

Lenalee Lee was one of the first people who noticed Allen's arrival—over the camera. The Black Order guys were suspicious of him, because of his strange appearance.

The boy

Allen noticed the camera, and tried to get in touch with the people inside. He said that Cross told him to come there.

the girl

She told her brother, the Supervisor Komui Lee, that their visitor mentioned about Cross. However, Komui said that he didn't hear anything about it. The people saw that Timcanpi was with this stranger, so it could be true that he was sent by Cross. They decided to let the Gatekeeper "scan" him.

The Boy

The Headquarters' Gatekeeper scanned him. However, it suspected him because he had a pentacle on his forehead—which usually indicates an akuma. Allen panicked.

The girl

Gatekeeper's decision caught everyone's attention, assuming the stranger to be a bad guy sent by the Millenium Earl. Lenalee then realized that Kanda Yu had already flown to the gate, to confront their suspicious visitor.

The boy

He had a hard time when exorcist Kanda Yu showed up in order to eliminate him. He activated his Innocence when Kanda tried to attack him. Of course, Kanda was surprised upon seeing that the "bad guy" bear an Innocence. Allen said that his master should have sent in a letter of introduction to Komui.

The girl

The officers finally found the said letter. Komui then assigned Lenalee to meet the newcomer.

Boy Meets Girl

Lenalee met Allen at the entrance and stopped Kanda from hurting Allen any further.

Lenalee: "Geez. Get in!"
(Allen, Lenalee and Kanda entering the gate)
Lenalee: "My name's Lenalee. I'm the head officer's assistant."

Lenalee shortly introduced herself to Allen, while Kanda simply said that he was "not willing to shake hands with a cursed person". Lenalee tried to make Allen feel better by saying that Kanda had just arrived from a hard mission. She then guided him towards Komui.

Komui performed an operation to "repair" Allen's injured arm. This is where we see that, in the beginning, despite Lenalee was being nice, she was also uncertain about Allen's identity. While her brother was repairing Allen's arm, she asked:

Lenalee: "Nii-san, are we going to check whether Allen-kun is human or not?"
Komui: "Hm? No, it's fine. He's human."
Lenalee: "How did you know?"
Komui: "Because the only being that gets cursed is human."

I believe she asked like that not because she was idiotic or something—in fact, Lenalee is the smartest out of the four exorcists (even compared to Lavi, the bookman). She wasn't talking behind Allen's back, either. He was there, probably almost passing out due to the shock of reparation, but he was there, together with her and her brother. Lenalee was most likely being cautious, thus she tried to confirm his identity; that was all. After this conversation with his brother, she didsn't seem to be thinking that way anymore—she trusted him and they became friends.