Allen is a beagle with brown and white fur. He is dressed in a star-shaped ruff, and a small top-hat on the right side of his head.
Allen seems to be a friendly and outgoing dog, as demonstrated when he licked Red's unusual hand just after meeting him.
Allen is also very playful and seems to enjoy light-hearted teasing, as shown when he steals Red's ball and drives the boy crazy as he chases him around. However, Allen is in no way mean-spirited, as when Red gives up, he licks him to soothe him and gives him back the ball.
Allen used to belong to a circus performer. When his master fell ill and died the ringmaster threw him out but the dog kept coming back. Hearing this story, Mana saw himself in the dog so he befriended him and took him in. 
Allen was Mana's pet dog who accompanied him on many circus shows. Mana is the one who named him Allen, which was a name that he remembered from his former life, which was probably one of his last memories of that time. 
The two first met at a circus, where a member of the troupe explains that Allen used to perform with them, but his owner had fallen ill and died. No longer having a use for him, they had Allen thrown out; but the dog kept returning, mostly likely in hopes of being fed.
Seeing the downtrodden dog sitting in the rain, Mana instantly feels a connection with him; he knows all too well the feeling of losing a loved one, and with that, he asks Allen to join him. Mana tells Allen that he too has someone he would like to meet again, and suggests that the two look for those they wish to meet together. With that, Mana and Allen walk off together.
Allen and Red meet one day after the dog sees Red pulling heavy meat buckets for the lions. Later, Allen appears as Red is eating next to a box of circus props. The dog steals a ball, forcing the boy to chase him around to get it back. When Red gives up in exhaustion, Allen comes back and provokes him, easily avoiding all the boy's assaults. When Red falls to the ground, Allen licks the bruises on his face and leaves.
Eventually, fueled by jealousy and rage, Cosimov attempts to feed Allen poisoned food, however Red almost immediately becomes extremely suspicious of it, warning Allen not to eat it. A fight breaks out which ends up with Red being knocked unconscious by Cosimov. Allen retaliates by biting Cosimov, but the altercation ends when Cosimov brutally punches him.
Some time after the altercation, Red wakes up in a panic and immediately tries to find Allen. He frantically runs in search of the dog, and is soon relieved to see he'd just finished a show with Mana. Red thanks Allen for his help, but the two are interrupted by Cosimov being scolded by the Ringmaster for not being able to perform properly. Cosimov is enraged over what Allen did to his arm, swearing that he will make Allen pay.
Concerned, Red tells Allen to stay away from him, and to make sure to stay with Mana at all times. When Allen licks Red's cursed hand, Red hugs the dog tightly before allowing it to run back to its master.
Despite this, the next morning, Allen is found dead.
When Mana is confronted with the sight of the body of his beloved dog, he tries his best to not show his grief. Red bluntly asks if Allen is dead, and immediately suggests that it was Cosimov who was behind it. Seemingly calm, Mana reassures him that Allen was an old dog, and probably would not have lived much longer anyway.
Red asks Mana if he is sad, and this causes Mana's happy facade to waver, his lips quivering. Despite this, Mana once again disguises his grief with humour, as he pretends to hang himself. He then explains that he cannot cry, and that his tears dried up long ago. Red is not convinced, however, as when he looks at the older man's smiling face, he concludes that he is actually crying.
Red does not understand how Mana can smile when he is sad, or how he is not angry; Allen was a friend who was supposed to travel with Mana forever. While he desperately tries to hold back from caring for his new found friend, Red asks for the dog's name. But he breaks down and tears flow from his eyes. Upon seeing this, Mana thanks him for grieving for Allen, and holds him in a tight embrace. It was then that Mana realises that Red was Allen's friend.
Soon after, the two began travelling together. With his sanity slipping, Mana started mistaking Red for his late dog, and started to call him Allen. From then on, Red adopts the name Allen.
Disappearance of Cross Marian arc
Before meeting with Cross Marian, Allen Walker thinks of his past encounter with Mana back when they discovered Allen's body. 
Searching for A.W. arc
- Allen's design was based on a beagle named Karen, which Katsura Hoshino had met. Unfortunately, Karen passed away in January 2019. 
|Pets||Timcanpy • Ur-Campy • Fat Cat • Allen • Lau Shimin • Atuuda • Gamako|
|Major Characters||Mana Walker • Jean • Guzol • Lala • Anita • Mahoja • Inspector Galmar • Mother • Barba • Katerina Eve Campbell • Sister|
|Minor Characters||Charles • Britannic Police Inspector • Marc • George • Eeez • Crack • Momo • Tricia Kamelot • Broker Doctor • Broker Nun • Cosimov • Joe • Lucia • Sophia • Riza • Colette • Jerome Dreselle • Serge Dreselle|
|Bit Characters||Fan • Kokona • Kokona's grandfather • Unnamed Lady • Unnamed Gentlewoman • Fourteenth's Unnamed Supporter • Sophia and Angela's Father • Sara • Madame Dreselle • Jaime Dark • Mister Martin • Miss Martin|
|Filler Characters||Victorio • Sandra • Claudia Sardini • Mister Sardini • Viscount Baretti • Mei-Ling • Elda • Frey • Narain|