Big Bad: The Sirens are the source of the Mental Cubes which give birth to the Kansens/ship girls.
However, there are some normal shells in their mix, and Oceana's most devastating attack is her close-range purple HE barrage.
Armor-Piercing Attack: Most Siren attacks are blue armor-piercing shells, which can do massive damage to your heavier ships.
Animal Motifs: All the Sirens are designed around marine life, such as various types of cephalopods, stingrays, and hammerhead sharks.
This would push the Sirens to Well-Intentioned Extremist territory since they are manipulating humanity for the sake of evolution against a greater threat.
The conversation between the Inquisitor and the Siren Observer Zero implies the existence of a third party whom humanity cannot defeat, even with the power of the Sirens.
It is clear that the Sirens seek evolution and growth above all else, but as for why, there are multiple possibilities shown:
Ambiguously Evil: The Fallen Wings event implies the motivation and purpose of the Sirens might be much more complex than they seem at first.
They leave humanity mostly to their own devices in the main game's storyline, but they do things differently in the events, in order to map out and see the various possibilities and consequences of their actions.
Alternate Timeline: The Sirens abuse this trope to perform their Reenactments.