Well for me there are a lot of reasons why Maple is my favourite Girl:
1. She is a Swordfighter fighting in Japanese Armour.
2. She has good Abilities
3. She has a very pleasant voice ( i personally hate Girls with high-pitched voices. My ears are bleeding by listening to some Girls)
4. She has an interesting backstory in my opinion.
5. Her Attack Animation is awesome.
6. She is sexy.
I think most guys who love Mature Women also love Maple. People who prefer Lolis won't appreciate Maple i guess.