Sienna's flame dagger, whilst small, is very effective in combat in the hands of the capable - notably sacrificing power for speed.
The Charge Attacks deal damage-over-time that can stack upon a successful hit, with each tick dealing damage per second for two seconds. The first charge doesn't Cleave very well, but it will still inflict the damage-over-time effect, while the second does hit multiple targets and deals damage to them. The second charge attack is also very effective against Armoured units, especially when a headshot is scored - ideal for Stormvermin.
The dagger, as shown in the tooltip, has a very small effective block angle. Blocking any attack outside the 40° arc in front of her will cost significantly more Stamina than usual, so caution should be used.
Her Push Attack is rather unique in this regard. Besides the push itself costing only 1 stamina (half a shield) to execute, it also pierces armour/shields to a degree, and can be effective for scoring headshots on tougher Enemies. This attack also inflicts damage-over-time, but to a much higher degree - dealing more damage per second for four seconds this time, as opposed to two seconds.
A video showing the attack-pattern of Sienna's dagger.
Damage over Time, Crowd Control, Fast Attacks