** Remember to LIKE the video to support the series! **
Comments
The Servant summoned is never random. The Masters have control over which Servant they summon by using a catalyst that is related to said servant. For example: Kiritsugu summoned King Arthur by using the scabbard of Excalibur as a catalyst. Others used different relics related to the servant they want to summon.
If there is no catalyst used, like with that murder psycho Ryuunosuke, then the Grail uses the personality/nature of the Master, to summon a Servant that's similar to that.