The Kaddish is not a prayer requesting anything for the behalf of the deceased. Rather, it represents the acceptance of God's will even in the face of loss, and continued relationship with the community despite the grief of the bereaved. It is equivalent to praying "the Lord giveth, and the Lord taketh away. Blessed be the Name of the Lord".
Thanks. But the El Malei Rachamim: seems to be asking God for something, doesn't it?
Exalted, compassionate God, grant perfect peace in Your sheltering Presence, among the holy and the pure who shine with the splendor of the firmament, to the soul of our dear ______ who has gone to his eternal home. Master of mercy, remember all his worthy deeds in the land of the living. May his soul be bound up in the bond of life. May his memory always inspire us to attain dignity and holiness in life. May he rest in peace. And let us say: Amen.
Do you know, roughly, when this practice began? Was it extant at the time of Jesus?