How can I mirror my weight painting to the opposed side of my character? I know that there is Mirror function, but it doesn't assign the weight to bones on the opposed side. Can I do it manually?
Each pair of bones need to have the same name with the suffix .L for the left side and .R for the right side. E.g. UpperArm.L and UpperArm.R. In the weighting Options of the tool panel turn on "X Mirror".
The mirror modifier takes weights into account:
In Weight Paint mode you can edit the weights they would be applied symmetrically.