Indra
Name
Like most of the avatars in the game, Indra's name comes from Hindu mythology. Indra was once one of the most important gods of the Hindu pantheons, but he was gradually replaced by Vishnu and Shiva until he became a minor deity.
Indra was the king of the Devas (the "good" Hindu gods), the son of Dyaus and Prthivi, god of thunder, storms, and the greatest of warrior; as a Lokapala, he guarded the East. However, he was also described having many human flaws: the most notable for this shrine being that Indra loved to drink Soma (a sacred hallucinogenic drink) to gain strength, or often just to get drunk. He is most famous for his battle against the Asuras (the "bad" Hindu gods) Vritra, a demon who had stolen all the world's water. It was after his victory over the beast that the gods elected him king as thanks. In the game, Indra doesn't live long enough to repeat his mythical exploit, sadly, which kind of sucks since since he's attuned to Vritra's weakness.
Appearance
Indra is, to be honest, hard to describe, which is why I provided a picture. Like most of the demons, he is human shaped. His skin is brownish-red, with purple lozenges; in places, his body seemed armoured with light brown plates with thin oranges lines all over them. He wears the vajra on his right arm, or rather the weapon is his right arm; he also wears a mask that makes it seems he has eyes, but when it opens to reveal Indra's faces, one can see that, like all the other playable demons, he has none.
Weapons
In the myth as in the game, Indra uses the vajra (thunderbolt) weapon to vanquish his enemies. In the game, though, the weapon is part of his arms, and several retractable blades are attached to it.
First Transformation
Roland transforms into Indra for the first time at the beginning of the Meat Factory—you can use him beforehand, as he can be put into your party after you meet the mole and there are some random encounter in the factory's entrance, but this transformation is a scripted one.
The Embryon walk in on two soldiers in the Meat Factory who've found one of Roland's moles and are about to execute him, thinking out loud that they were lucky they found him before "the Boss" did. Roland says it doesn't matter, since they'll take care of them. The soldiers turn toward the voice, and recognize... the unidentified Tuners standing next to Roland (you'd think the Society would know the face of the rebel leader). They both turn into Nues and get ready to attack you. Roland asks to be allowed to handle them, as "[he] could use the practise." As he slowly raises his marked hand to push his glasses up, looking intently at the soldiers and looking very badass in the process (see the middle screenshot in this site's header), he transforms into Indra, retracts his blades, and opens his mask to roar at the Nues in challenge.
The Nues don't don't seem too keen on having to face Indra, as they probably didn't expect Roland to be a Tuner too, but after a moment they nonetheless charge, and the battle begins as Roland is forced into your party lineup by the game.