Some relevant skills, off the top of my head:

• 'Mind-reading', e.g. how are they going to interpret a complaint or request or comment
• 'Filtering', e.g. what decision or evaluation should or must be made by a 'hero'
• 'Readiness', e.g. "this just needs your signature"

Really, there must be lots of specific sub-skills, as the three I listed overlap to a large degree.

I initially thought I would be able to list lots of skills specific to software, and of course there are many, but they're relatively unimportant for being a good sidekick generally. For example, being able to provide clear instructions on how to reproduce a bug is incredibly valuable, but that's really just an example of the general skills I listed above, i.e. providing info that's unambiguous about what's wrong (and ideally why), not providing info that's irrelevant, and providing enough info so that they can most efficiently fix the bug.

Generally, being a good sidekick requires sufficient empathy and self-awareness. Empty because you have to know the mind of your hero to know how to best help them. And self-awareness because you have to know whether your hero's cause is really yours too. Tho, of course, some sidekick's cause is ultimately serving a specific hero.

In fantasy terms, a good sidekick delivers obvious monsters that the hero can slay.