Sorry you didn't get the specific diamond you were looking for. Unfortunately we can't guarantee the results of any relic opening - that's the nature of random relics.
The contents of every Relic in Skullgirls is dictated entirely by the loot table for every Relic. To see the reward odds for each Relic in game tap on the (i) icon on the upper left of any Limited Time Offer, or on the upper right when viewing Relics. This will list the percentage chance for each reward a Relic may contain. Understandably, Diamonds have very narrow chances to be pulled.
However, there are a bunch of systems in place to help get the characters you want. Right now there is a 2x chance to pull Robo Fortune from relics, so if you can manage to build up a free Diamond Relic by collecting shards, that will be your best chance to get an Overclocked.
