The same as hair carrying an electrostatic charge

If mana is stored on hair, the mana on each hair will repel the hair next to it. As Serena charges up her magic, her hair will naturally billow out into a perfect puffball. In other words, she will become a lady with thistledown hair. The pressure between hairs provides a pressure of mana, held in place by her will, which can be released like letting air out of a balloon. Long straight hair could be an issue though, requiring brushing and regular maintenance. You're also limited by how long your own hair is.

The next step would be braiding and hair extensions. Braids do not need as much daily maintenance to stay braided, and hair extensions allow you to make your hair as long as you want. Individual hairs will not store as much mana, but the increased length and mass of braids could let you push more charge in overall.

The "puffball" now becomes something looking more like a pincushion, which is an awesome look on the battlefield. Imagine tribes of horsewomen with 1-metre braids flared out from their head, charging the enemy as lightning flickers around the tips of their hair. That's a movie I want to see!

It becomes less practical indoors though. Charge yourself up and you'll burn the ceiling, walls, and anyone near you. There is a solution though - braid beads into the end of your hair. It'll take much more mana to lift the beads against gravity, meaning that you can get away with a much shorter hairdo. However you need to be able to push much harder to charge them up, and not every magic user is strong enough to do it. So if you see someone coming down the street with big weights swinging on the ends of their braids, be very very polite to them!