BASICALLY: In this world, Yuu Kanda is the one who became the experimental womb for the Third Exorcists, not Alma Karma.
Fastforward to canon, and everything else is pretty much the same--except instead of the grumpy, silent Kanda, you get the bouncy smiley Alma.
Obviously some things are different, Alma still threaten Allen when he showed up, but it was with a smile and a cheerful apology afterwards.
Alma has a lot of the same habits as Kanda, but a very different surface personality.
IN MORE DETAIL: For the most part, things happen the same as in canon, Alma does the same things Kanda does, except for different reasons and with a different attitude.
For example, Alma still attacks Allen when they first meet, however, once everything is sorted out, Alma apologizes and cheerfully introduces himself to Allen.
Another example is that in the town of Mater, Alma still insisted on taking the Innocence from Lala and was totally perplexed by Allen's refusal to do so. Alma still injured, and was still very upset at Allen's misuse of the Exorcist coat.
However, Alma wasn't threatening to Allen or outright mean. Alma is set on getting missions done correctly and is quite proud of being an Exorcist, however, anyone outside of the Order he doesn't really care about. Order members are his family, the only ones who matter to him. An old man and a doll weren't people he cared about, until Allen made him see that they were important as people too.
Alma is very lighthearted and friendly on the surface, but underneath that he is pretty much the same as Kanda. He's very attached to the other Exorcists, and to the Order--it's his home, his only home. When not being annoying and overtly friendly, Alma meditates and enjoys his meals of soba (unlike Kanda, he does eat other things). He spends a lot of his free time training, but not as insanely much as Kanda would.
Unless the new canon says otherwise, Alma has Mugen as his Innocence.
PHYSICALLY: Alma Karma (looking at womb/Akuma!Alma in the new chapters anyway) is just as tall as Kanda, and just about the same age. They have a similar build and similar strengths. Alma keeps his hair shorter than Kanda, and has thick eyebrows and a scar/mark across his nose.
RELATIONSHIPS: Exorcists are family. Noah are bastards. The usual.
Yuu Kanda: /sob you are dead only not.
Allen Walker: Beansprout! rather friendly with, but cautious of since he's cursed.
Lenalee Lee: good friends with, and have been friends since they were young.
'Lavi' Bookman Jr: still finds him semi-annoying, but they tend to play pranks together.
Baron Arystar Krory III: weepy vampire-man.
Miranda Lotto: pretty cool but could use some self-esteem
Bookman: Creepy old man.
General Froi Tiédoll: sob you're sorta creepy father figure.
Noise Marie: BRO!
Daisya Barry: BRO!