[@Lucius Cypher] While the thought of Orchid of all the people getting all sneaky amuses me, I'll go with the default multiclassing rules discussed on PHB p.163. This would mean to get Rogue levels Orchid needs a score of 13 in STR (to fulfil the requirement of his current class, which is easy) and another 13 in DEX (to fulfil the requirement of Rogue, which they also have). In the end, if they have acted in somewhat rogue-ish manner, you can take a level in the class to show their advancing whenever you would gain a level.