Shin Megami Tensei

Shin Megami Tensei is a multimedia franchise created by Altus. Originally based on a novel about demons in the internet, the series has expanded to include all manner of settings, from post-apocalypia to pre-WW2 Japan. Demons are still a constant in the series, and the protagonists can usually make use of them by either capturing or making deals with them.

The series title translates to 'Resurrection of the True Goddess', although who or what the goddess is, and whether or not the goddess is on the same side as the protagonists, changes with each game.

Arachne
Arachne is a recurring Chaos alignment demon, and a boss in SMT.

Alraune
The Alraune is a recurring Dark alignment demon.

Baal
Baal is a Good alignment boss in MT2.

Banshee
The Banshee is a recurring Neutral alignment demon.

Beelzebub
Beelzebub is a recurring Chaos alignment demon and occasional boss. Usually depicted as a giant fly, it is ocasionally depicted as a woman.

Bushyasta
Bushyasta is a boss in SMT if....

Chiaki Tachibana
Chiaki Tachibana is a friend of the player avatar in SMT3. After the Conception she struggles to find her place in the world of demons. She eventually claims the power of a powerful dead demon to become a demon herself, and attempts to use this power to forge a new world based only on strength. Whether or not the player sides with her she eventually dies, although the player can choose to follow through and create her world.

Druj
Druj appears as a boss in SMT if....

Dryad
Also known as Daphne. The Dryad is a recurring Light alignment demon.

Echidna
Echidna is a boss in SMT. In mythology she is the mother of most classical greek monsters.

Empusa
Empusa is a recurring Neutral alignment demon. In mythology Empusa was a malevolent spirit in the shape of a beautiful woman, although occasionally with the legs of a horse, who would trick men into bed and then eat them.

Ganga
Ganga is a recurring Light alignment demon.

Harpy

 * See article: Harpy

Hecate
Hecate is a boss in SMT II.

Heqat
Heqat is a Chaos alignment demon.

Jezebel
Jezebel is a biblical name usually given to women of ill repute. She appears as a boss in Devil Survivor.

Jorogumo
The Jorogumo is a traditional Japanese youkai who surprisingly has not seen a lot of face time in the MegaTen series.

Kiyohime
Also known as Otohime and Mermaid.Kiyohime is a recurring Neutral alignment demon.

Lamatsu
Lamatsu is a Chaos alignment demon.

Lilith
Lilith is a recurring demon, and boss in SMT and SMT if.... This Lilith incorporates a snake into her design, unlike the Succubus-style Lilith which is more generically demonic.

Melusine
Melusine is a boss in Shin Megami Tensei if....

Naga

 * See article: Naga

Nekomata

 * See article: Nekomata

Pixie

 * See article: Pixie

Princess Inaruna
Princess Inaruna is the final boss of SMT: Devil Summoner.

Scylla
The Scylla is a recurring Neutral - Chaos alignment demon.

Serket
Serket is a recurring Neutral alignment demon. In mythology she is a benevolent Egyptian god of venomous animals.

Siren
The Siren, in a rare aquatic design, appears as a boss in SMT if....

Sphinx
The Sphinx is a recurring Neutral alignment demon.

Students
In Giten Megami Tensei some of the students become possesed by demons. Or maybe they were demons from the start. There's very little english info on GMT, so I haven't the foggiest.

Succubus

 * See article: Succubus

Tiamat
Tiamat is a recurring Chaos alignment demon, and a boss in SMT II. In mythology she is a creation goddess and mother of many gods and monsters. After she was killed parts of her corpse were used to create the world and the heavens.

Undine
Also known as Rusalka. The Undine is a recurring Light alignment demon.

Wyvern
The Wyvern is a recurring Neutral alignment demon.