Rell

Rell, a Tenno lost to history, who was outcasted by Margulis. He was one of a number of children that was on the Zariman Ten-Zero, However, unlike the other children Rell had difficulty identifying emotions. his mother, Kay was an adult aboard the Zariman who perished in the onslaught. before her madness, she helped Rell to understand and recognize emotions using a deck of cards with faces drawn on them. During the Quest The Chains Of Harrow you can hear audio clips of how she would help him understand emotions.

In the quest Chains of Harrow he wrote messages on the walls of corpus ships in blood presumably to the Lotus whom resembled Margulis, the Archimedian who refused to take in Rell like she did the other Tenno. This led to Rell to be alone until the Red Veil syndicate was formed, where the founders took in Rell so they could study his teachings.Though in order to give Rell such a long life, instead of sleeping in a Cryopod  - like the other Tenno were for generations - his consciousness was transferred to his "undying vessel" we now know as the warframe Harrow and then he asked his followers to chain the warframe up.

Donda
the Donda was a device given to Rell, most likely by his mother that he used to focus his thoughts and to deal with the sensations of sound, colour and touch which sometimes overwhelmed him.