Imperial Knight pilots as PCs? I don't want to bring an actual Mecha into the game but what about a pilot who lost their Mecha? Or was the second son and didn't inherit it but got the training? Or flunked out of training? What would the attributes be? Skills? Could they just plug into most vehicles?

One of my players had been contemplating playing a freelance Knight who'd lost his mecha, but he ended up going in another direction. If he decides to revisit that concept; I'd just recommend that he go Highborn / Outcast / Ace, and trade in a couple of equipment choices for a Mind Impulse Unit.

As for plugging into other vehicles, I'd limit it to what anyone else could do with an MIU.

Highborn or Feudal World Ace would probably be best, yes. Not sure what the best Background would be...

A disgraced Knight does seem like a compelling character; I've always liked stuff like that back when I was into Battletech.

Agreed. Highborn/Outcast/Ace seems like a good model for the character. Feudal World might be relevant too - aside from their knights, most Knight worlds are low-tech.

As to which vehicles they can 'plug into' - any powered vehicle theoretically can be fitted with MIU gear. Not every vehicle will be.

I'd pointedly take Operate (Ground) and then relatively 'noble' skills - scholastic lore (heraldry), if it's still about, is very apt, as are any influence-related skills.

I'd do a bit of mental charting about the house he is/was from - Knight houses tend to be either Mechanicus or Imperial aligned - I would expect common lores to suit - a mechanicus house wouldn't have common lore ecclesiarchy, for example.

A second son having the training but no suit is quite feasible. Many less influential houses only have one or two suits. The traditional way to increase this is to join an imperial crusade - a forge world provides the house with additional suits, the house supplies the pilots, both go off and merrily kick xenos arse, and the force later comes home - hopefully still with more suits than it originally set off with.

Of course, if there's no mechanicus-sponsored crusade nearby (or at least none the imperium will admit to!), or the forge which traditionally supplied the houses' knights has been eaten by the pandaemonium, or whatever, then a scion with no realistic access to a knight suit is quite possible.