"A roadie's job is to make someone else look good." Eddie's most-important function on the battlefield is to double-team with other units and significantly increase their killing power. Your band is significantly weaker without vital Roadie support!
Especially significant when you double team with Lars. Alone, Lars is... well, not bad, but definitely weak for a hero unit. With his trusty roadie to help him, he cuts through the Tainted Coil like a fat kid through cake.
The other commanders' Double Teams reflect their group's essential character. Ophelia's double-teams always involves some kind of depressing act (being buried by Grave Diggers or dancing with a Bride in the rain) while never thanking her minions' efforts. They possess a measure of kinship, but they are too busy suffering a goddess' sorrow to really enjoy it. Doviculous kills, tortures, breeds with his children, or is harmed himself. He does these things because that is what the Tainted Coil is all about, and as the absolute ruler of the Coil it is his divine right to do so.
During the fight at the dry ice mines, Ophelia and Eddie exchange a few words...
Eddie: Ophelia! My father Riggnarok — is that the secret you couldn't tell me?
Ophelia: What about him?
Eddie: That he was, you know, your parents' boss.
Ophelia: Idiot. You don't know anything... including how to fight!
The way she says "what about him" sounds bored enough that it's easy take it as sarcasm the first time around. But once you know the whole truth, it takes on a different meaning. Ophelia was testing Eddie: since Riggnarok is half of the answer, she wasn't really sure if he'd figured it out or not. Once he answers her question, she realizes that he still hasn't put the pieces together and tells him so.
Eddie claims to have dreamed about flying since he was a kid. Around the time he gets wings.
He also apparently got shot in the back during an earlier fight. He collapsed to the ground in pain, Ophelia checked his back and found nothing. At the start you could chalk this down to the Healing Factor that each Hero Unit has in this game. The pain was his wings attempting to push out ahead of time.
Why do the seagulls affect the speakers at the Screaming Wall? It's because they're microphones.
Why does Lars' sword have such a strange shape? Look at the sword Riggnarok carries in the Legend clips. Lars' sword is clearly part of Riggnarok's broken blade.
And still the scabbard matches the broken blade!
So, Roadies have invisibility powers. Now compare real-world roadies to the Japanese stagehands listed under Real Life examples of Highly-Visible Ninja. Yup, that's right, they're the same thing in different genres. The origin of the Ninja stereotype is the Kabuki equivalent of a roadie.
Eddie pledges to finish his father's quest to kill the emperor, not realising that he's already done it, since that particular emperor died giving birth to Eddie.