Helen is the second recruitable boss in the Trojan Warriors Campaign.
Unit Type - Priest: Bless a random friendly gambit hero. Spell - Blessing: Attempts to raise a random friendly gambit heroes morale to 100.
Helen is a higher tier version of Boudica. She has better bravery, similar tactics, higher magic and a better unit count.
If you're willing to put the time into it and are nowhere near your kingdom's prestige hero priest, then by all means train her up as soon as you can beat her.