In spiritual realm bijaksharas have a vital place. Many mantras carry bIjaksharas, apart from the deity's name.
For example: chamunda mantra
“Om Aim Hreem Kleem Chamundaye Viche”
In general Aim bIjakshara is related to Saraswati. In Rig Veda, Saraswati Suktam appears, wherein Saraswati was eulogised as eliminator of majic spells and demons, as Indra did.
aindra indicates that related to Indra.
As per this answer, the bijakshara Aim - (ऐं) related to Indra.
My doubt is
If aim (ऐं) is related to both Indra and Saraswati, then how are Indra and Saraswati related to each other?