Fire Mei (Mei)

Mei the Fire Martial Cat is a fusion monster for the great Baretta but in early game could be a good kickstarter for Arena defense/offense (offense only as a trap against water units) and dungeons, her second skill puts her in automatic counter attack while increasing CR% thus making her a great help with low floor bosses.
Although her monster type (attack) her base atk stats are really bad until maxed and awakened, so focus on runes and give her food (do not use devilmons!) to get the most out of the third skill, which is a greatful help for Giants and Arena slow teams and you can also team her up with Fire Magical Archer (Cassandra) to gain a 35% attack bonus.

In conclusion, build her 4 stars maximum because you'll like to get a better alternative in the near future! She is too squishy and requires a good healer in the team to stay alive.

Mei, Fire Martial Cat Suggested Runes

Early and Mid game: Fatal/Blade (Atk%/CD%/Atk%) with Spd and Hp% on substats