Siren enemy line

The Siren, Succubus and Nightmare are species of monsters found in Golden Sun and Golden Sun: The Lost Age. These monsters, resembling pale-skinned human females with long hair, are palette swaps of each other that have differing levels of power and defense and are fought in different locations in the games.

Siren
A Siren is an orange-haired variant with a light-green dress and purple boots. Statistically, this monster has 116 HP, 12 PP, 107 Attack, 27 Defense, 54 Agility, and 9 Luck. In terms of resistance, it has a Mercury Resistance rating of 127, Venus and Jupiter Resistance ratings of 100 each, and a Mars Resistance rating of 72, and in regards to its abilities it uses its available Mercury-based attacks with a Mercury power rating of 100.

A Siren can use the following battle commands:
 * Attack: Used 2 out of 8 times, this is this monster's standard physical attack.
 * Froth: Used 2 out of 8 times, this is a Psynergy spell that summons a small array of watery spheres that assault the Adepts, dealing a Mercury-based attack with a power rating of 28 and a range of 3. It consumes 5 of its user's PP
 * Sleep: Used 2 out of 8 times, this is a Psynergy spell that affects up to 3 Adepts, with each one having a chance to get afflicted with the Sleep status ailment as a flock of sheep falls around at their positions. It consumes 5 of its user's PP.
 * Heartrender: Used 2 out of 8 times, this Monster Skill has the battle text read "Siren glowers miserably at you” as a group of glowing pinkish-white hearts converges on a single target, lowering their attack power by 25%.

Felling a Siren yields 96 Experience Points and 127 Coins, and there is a 1/16 chance that the monster will randomly drop a Crystal Powder. If it is felled with the attack effect of an offensive Mars Djinni, its rewards increase to 124 EXP and 165 Coins, and the chance it will randomly drop its item becomes 1/4. The Siren is only fought throughout in Mercury Lighthouse.

The Siren certainly qualifies as one of the standout monsters battled in Mercury Lighthouse, having the highest Agility rating and slightly outperforming the Harpy in all stats. Only the Lizard Man has higher HP and Defense. Therefore, it is these monsters you should generally focus your attacks on throughout the dungeon.

Succubus
The Succubus is a blue-haired variant with a turquoise dress and violet boots. Statistically, this monster has 529 HP, 56 PP, 195 Attack, 49 Defense, 101 Agility, and 12 Luck. In terms of resistance, it has a Mercury Resistance rating of 127, Venus and Jupiter Resistance ratings of 100 each, and a Mars Resistance rating of 72, and in regards to its abilities it uses its available Mercury-based attacks with a Mercury power rating of 100 and its available Jupiter-based attacks with a Jupiter power rating of 85.

The Succubus can use the following battle commands:
 * Attack: Used 2 out of 8 times, this is this monster's standard physical attack.
 * Hail Prism: Used 2 out of 8 times, this is a Psynergy spell that causes four large ice spheres to slam into the party of Adepts and shatter in quick succession, dealing a Mercury-based attack with a power rating of 90 and a range of 5. It consumes 16 of its user's PP.
 * Wind Slash: Used 2 out of 8 times, this is a Psynergy spell that projects a moderately wide glowing purple swath of energy through each of multiple Adepts, dealing a Jupiter-based attack with a power rating of 50 and a range of 3. This ability consumes 9 of its user's PP.
 * Heartrender: Used 2 out of 8 times, this Monster Skill has the battle text read "Succubus glowers miserably at you” as a group of glowing pinkish-white hearts converges on a single target, lowering their attack power by 25%.

Felling the Succubus yields 101 Experience Points and 143 Coins, and there is a 1/16 chance that the monster will randomly drop a Crystal Powder. If you happen to fell it with the effect of an offensive Mars Djinni, its rewards become 131 EXP and 185 Coins, and the chance it will randomly drop its item becomes 1/4. In Golden Sun only one Succubus is fought throughout the entire game, together with two Grislys as one scripted boss encounter that occurs on the second "dungeon floor" of the optional Crossbone Isle as soon as you try to enter the door to that floor's puzzle room. Therefore, the minimum rewards of the battle are 587 EXP, 1543 Coins, and a single guaranteed Potion.

As part of a team of three monsters that is one special encounter, the Succubus has comparatively few HP but is the user of area of effect Psynergy for elemental damage, while the Grislys are the monsters that are strong and durable. This battle should only be adequately difficult by boss battle standards if Crossbone Isle is visited and this battle is fought earlier in the game through the hidden method at Tolbi-bound Ship. This is definitely a battle that can be won in one turn by having everyone use full-powered summonings at the start, but if you decide to battle these the typical way, the Succubus is the target that goes down much more easily than the Grislys, so that should probably be focused on first. Mars-based Psynergy is what all three monsters are weak to.

Nightmare
A Nightmare is a violet-haired variant with a dark-purple dress and light green boots. Statistically, this monster has 258 HP, 28 PP, 287 Attack, 89 Defense, 138 Agility, and 13 Luck. In terms of resistance, it has a Jupiter Resistance rating of 127, Mercury and Mars Resistance ratings of 100 each, and a Venus Resistance rating of 72, and in regards to its abilities it uses its available Jupiter-based attacks with a Jupiter power rating of 100 and its available Mars-based attacks with a Mars power rating of 85. It should be noted that its resistances and elemental power ratings are different from its earlier counterparts.

A Nightmare can use the following battle commands:
 * Shine Plasma: Used 2 out of 8 times, this is a Psynergy spell that causes three purplish-white lightning bolts to strike the party in quick succession, dealing a Jupiter-based attack with a power rating of 100 and a range of 5. It consumes 18 of its user's PP.
 * Fireball: Used 2 out of 8 times, this is a Psynergy spell that causes a group of fire balls to spread out from the user and strike the party, dealing a Mars-based attack with a power rating of 65 and a range of 5. This ability consumes 12 of its user's PP.
 * Bind: Used 2 out of 8 times, this is a Psynergy spell that displays a holographic symbol onto a targeted Adept, attempting to Seal that Adept's Psynergy capabilities. It consumes 4 of its user's PP.
 * Attack: Used 1 out of 8 times, this is this monster's standard physical attack.
 * Heartrender: Used 1 out of 8 times, this Monster Skill has the battle text read "Nightmare glowers miserably at you” as a group of glowing pinkish-white hearts converges on a single target, lowering their attack power by 25%.

Felling a Nightmare yields 241 Experience Points and 198 Coins, and there is a 1/64 chance that the monster will randomly drop a Healing Ring. If it is felled by the effect of an offensive Venus Djinni, its rewards increase to 313 EXP and 257 Coins, and the chance it will randomly drop its item becomes 1/16. In Golden Sun it is first battled at Suhalla Gate, and is subsequently battled in the frontal "exit" portion to Venus Lighthouse and in the Road to Babi Lighthouse. In The Lost Age the Nightmare can be fought in the area of Trial Road accessed by the Hover Psynergy.

In Golden Sun the Nightmare is the fastest of the monsters it is fought alongside in both Suhalla Gate and frontal Venus Lighthouse, which when considering its other solid-by-comparison statistics and its varied party attacks gives it its own flavor of threat to particularly low level parties. The Earth Golem is more or less its inverse, being the slowest but the toughest offensively and durably. In the limited area it is fought in in the second game, it is quite average.

Siren
Origin: Greek

The siren's exact description varies; while they’re usually depicted as bird-women, they are sometimes portrayed in later folklore as mermaid-like. The Wikipedia definition can be found here

Succubus
Origin: Europe

A Succubus is a demon who takes the form of a beautiful woman to seduce men in their dreams, while drawing energy from them to sustain themselves, often to the point where the victim dies.

Nightmare
Origin: Demonology/Multiple

A nightmare is a frightening dream that was originally believed in many cultures to have been caused by a monster, evil spirit, or demon that oppressed people in their sleep.