The usage of the word "secondary" actually makes perfect sense.I don't like the word secondary being used here.
The results of solo-Cloud players are either underwhelming, inconsistent or both. If you looked only at their results with no regards to the people that use him as a secondary character you probably wouldn't understand why Cloud is considered such a good character. M2K is inconsistent, Tweek still gets a lot of credit for results he doesn't have [yet?] and Rain's results have gone down the drain completely ever since he started to main Cloud.
So far Cloud has only worked out as a secondary character as a complement to somebody else.