Mercenary is good and all but I feel like going Fighter is better simply for weapon variety in the beginning. I mean let's really look at it, Chrom is swords, Robin has swords and tomes, Sully and Stahl have sworda and lance, Viriom is bows, Miriel is tomes, Ricken is tomes, Sumia is lances Kellam is lances.
Only Vaike and Feddy can use axes. And even after that Libra and Cherche, I'm not counting Basilo, Flavia, and the spotpass characters due to how late they come in, that's only 4 default axe users compared to 11 default sword users and 6 default lance users (not counting Donnel).
Axe users always seem to get the short end of the stick a lot in fire Emblem lol.
I still make him a Mercanary most of the time though because swords are cool. But whatever class you make him, he's not gonna be bad either way.