devoted exorcist.
forever and ever; the boy and his devotion
After his Innocence activated, Allen spent 5 years training with Cross to become an exorcist, and was ‘abandoned’ in India after he got hit with his master's hammer on the head. He was told to go to the Black Order Headquarters in order to become a ‘certified’ exorcist.
Allen didn't receive the best welcome ‘party’ upon his first arrival to the Headquarters, the gatekeeper mistook him for the Earl’s rat because of the pentacle on his forehead, and even another exorcist (named Kanda Yu) was ready to kill him because of that. But later on, Allen made really good friends with a number of people there.
raison d’etre
The most common reason to become an exorcist is probably one’s awareness of their own power and possibility to save humans, to save the world. However, Allen’s original intention is different.
Due to his sorrowful experience with akuma!Mana, akuma has become important to him. He realizes that the souls trapped inside the akuma are in despair, and they need to be saved. In addition to that, the curse on his eyes allow him to see the souls themselves, including their expressions.
I have to say, one of the things that I really like about Allen which make him different from other heroes from any animanga series is because of his concerns and sympathetic feelings towards his enemies (in this case, Akuma).
Unlike the other exorcists, when Allen defeats an akuma, he is often seen saying things like “Good night” or “You're welcome”. Whenever he says it, his expression looks like a mix of relief and sadness. The souls contained within the akuma probably can sense his good intentions, too, because we often get to see them thanking Allen for bringing them salvation. It’s almost as though Allen exist just for the sake of akuma.
Often, Allen’s sympathy towards his enemies overcomes his logics, and it kicks up a row between him and his exorcist friends. Hell, he threw himself to exorcise an akuma who’s on a self-destruct mode, just because they can only be saved (exorcised) by Innocence. Allen got hurt a little in his desperate attempt and the akuma exploded, he didn't make it on time because his friend Lenalee Lee tried to save him. “Because we're friends!”, she said.
Allen’s careless action, while showing how selfless he was, also displayed his conceited side at the same time. He wanted to follow his ideals of saving akuma, and wanted everyone to understand it. When he failed, he blamed his friend, who cared about him more than the akuma. The way he shouted at Lenalee back then implied how important an akuma was to him, and how he didn't realize that somebody cared about him.
To be honest, it was understandable because Allen had always been used to being alone (alright, there were Cross, Mana, and about two others... but there were barely anybody else). However, since he joined the Order, he met and befriended a lot of people. Some of them might be curious about Allen’s appearance at first, but they accepted him. When Allen comes back from his missions, there are always some people who greet him “Welcome home”, and it makes him feel nostalgic and happy. Eventually, he begins to think of the Order as his home. Those friends he met in the Order have become his new family, and he feels the need to protect them. He no longer thinks of only akuma, but also humans. Both are important to him—I believe that bickering with Lenalee partly made him realized how his world had grown. These feelings and realization make him stronger from time to time, for example, how he regained his Innocence back in China, or how he fought against Tyki inside the Ark. Later on, although Allen still has the tendency to risk himself, he's more careful and aware about other’s feelings, not akuma alone.
an oath.
On the first day he joined the order, Allen made a promise to Mana that until my very last breath, he’d keep walking and wouldn't stop no matter what. He will walk down the path of being an exorcist. However, he learns a shocking truth his master has been hiding about Allen himself a while after he joined the Order. That he’s not just exorcist—that he has the ability of a ‘Musician’, because he is implanted with 14th Noah’s memory. Allen begins to doubt the path he’s chosen, that someone else might have already determined the path for him. Some Black Order members also begin to doubt Allen, and want him out because they consider him a rat—he doesn’t belong to the Order.
In the most recent chapters, Allen finally decided to leave the Order. Lenalee caught up to him right before he left. She tried to stop Allen and reminded him the consequences of what he was about to do, but Allen only smiled and said:
“No matter what happens, I'll always be an exorcist. Even though the paths we choose are different... that will never, ever change.” — Allen Walker to Lenalee Lee, Chapter 205.
That marks the change of Allen’s status, to become an independent exorcist. Eventually, his comrades might have to fight against him, too.
Actually, before Allen came to the Order, he already had the abilities to be an exorcist with his left arm. It was possible for him to become an independent exorcist. Being independent would actually suit him better, considering how Allen only fought for the sake of Akuma. However, he decided to go with the Order because he would be able to gain more information and keep track of the Earl’s movements. And I think it was a good move. Allen probably didn’t expect that he would be accepted so much by his future comrades in the Order. In the end, he cherishes the moments he share with the Order family members. Even though they're apart now, Allen thinks of the Order as his true home. And just like he promises Mana, he’ll never give up being an exorcist.