"See I have set you as Elokim (literally "God", can also mean "judge") to Pharaoh, and your brother Aaron will be your prophet"
Hashem had to enclothe Moses at that point with the attribute of strict judgement, in order to bring harsh judgements upon Egypt. However Hashem told him to "see that I have set you", meaning to meditate upon the fact that I have set you for Elokim, meaning I have enclothed you with the attribute of strict judgement, that this is only in regards to Pharaoh, however Aaron your brother will be your prophet, meaning he will be a defender, and he will bring you to the attribute of mercy in regards to the Israelites. Because he needs now to say good things about the Israelites against you, in order to bring the attribute of mercy to the congregation of Israel.
(Divrei Binah - Parshas Vaera p. 36)