I always thought the bard hit above her weight class. She was the second-to-last character I picked up, but she went into my party lineup regularly pretty fast because she was so versatile. She was running with my L9’s as an L6.
The rare sword that floats was a good early weapon for her. It stuns enemies for 2 turns and hits pretty often which can give you breathing room to swat enemies. I managed to get hers leveled up pretty high and she kept using it until I lucked into her Legendary weapon (which is ridiculously awesome). I skipped her epic weapon because I never got it until after she hit L10 and could equip the Legendary.
The Epic lute gives Dominate. You can pick up epic card packs if you have the gems until you get it, or hit dungeons that have it as a reward. I prefer it over her Legendary lute (which I also lucked into pretty fast).
Her heal effect with her headgear is more effective than the cleric, at the expense of much longer cooldown. But it’s real useful when your tank is in trouble and needs a big boost of health.
All her leg gear is pretty useful (though I don’t have her Legendary yet). I’m using her Rare pants since they leveled past the Epic pants stat-wise, but the Epics are pretty good too.
Calliope is pretty much in my party permanently. She’s a great jack-of-all-trades that can fill in for the cleric in a pinch and can run with the spellcasters if you get a decent weapon. She does suffer from low health and AC, though - I pair her with Tommus and his 4-round Taunt to keep the heat off her. He takes hits (and counterattacks perpetually) while she pelts enemies with her floating sword. Plus I just think she looks cool.