As I have said many many times in this shrine which of course I will be mentioning more, Alphonse is kind in heart and spirit. We can say that he is the ideal hero in the story because of his personality of being compassionate, gentleman-like, strong, and humble. Scratch that. We should say that Alphonse is the perfect ideal hero in the story.
Alphonse is always found calm and composed standing beside or behind his tiny older brother, Edward. Edward usually throws tantrums around people who calls him shorty, and Alphonse would be the one to calm his brother or manhandle Ed so that he won`t pick a fight. Even if Alphonse gets angry, he would still manage to calm himself down and manage to make Edward calmed as well. Alphonse is the kind of person who forgives and forgets. For example, even though their father had left them without any explanation while seeing their mother's sadness from their father's absence, Alphonse never blamed their father for their mother's death nor was angry at him unlike Edward who hates their dad. Also, Alphonse would never blame anyone or neither think of blaming someone else. As a matter of fact, Alphonse blames himself for having an armored body and never blamed Edward at all. Even if it was Edward's idea to resurrect their mother in the first place and Alphonse just went along with his older brother. Kindness is his natural nature.
Alphonse is very compassionate and loving. He naturally wants to make sure that everyone of his friends and those around him would be happy and safe. He could never let an abandoned cat to be left alone even if his older brother forbids him, Alphonse will still pick it up under the rain and keep it inside his armor without his older brother noticing. Alphonse also has a talent of understanding people and making them realize that they should "never give up" and "always move forward" and "if there is a chance there is still a way". Like Zanpano and Jelso who are ex-military chimeras who was abandoned by Kimblee, Alphonse was able to help them realize that there is still a way to find their original body with his encouraging words.
You used to say you would never give up as long as there was a chance you could get back your original body, remember? Instead of leaving our problems to others, we need to make a move.
→ Zanpano and Jelso
Alphonse is strong and not just because he could fight big strong guys, but because Alphonse is as determined like his older brother Edward. Alphonse' strong resolve is to save all the people he can while he is still alive. Even if he was loosing a battle against Pride, Alphonse never back down and never left Mr. Heinkel who almost bleed to death. Alphonse also protected a friend, May knowing that he might die if he did. One of the things that I admire about Alphonse is that he never seem to see his life half empty. The fact that he couldn't sleep, feel, or eat with his armored body, Alphonse still finds hope by writing down the things that he wants to eat or do when he gets his body back even though having an armored body is "lonely".
Alphonse is also a gentleman. He is always paired up with a female companion in fighting if Edward is not present. He would strongly protect those female characters and would give them strong inspirational comments that would give them hope to try and survive. His over all warm and loving personality has also came from their kind mother.
I think the reason for Alphonse being so affectionate not only with cats but with dealing with people's emotions is because he desire for warmth and love. His armored body cripples him from what he desires and so this is also the reason why Alphonse unconsciously likes to pick up stray kittens.