The flavor is pretty cool, but I think the mechanics are quite weak here. Intrepid is in close contention with Expose Weakness for worst card in the Heart of the Elders pack. The benefit is pretty weak and you have very little control over it. At best, Intrepid is going to work out to around +4 scattered over a bunch of skill tests. Even in this optimal situation, that's still a fairly weak card. I would actually rather take Take the Initiative or even Unexpected Courage just to get a more substantial boost to a single test, which is going to be much more consistently useful and much easier to plan around.
Why do people think Streetwise is better than Keen Eye? Part of the reason is just that Rogues have more resources to spare than Guardians, but another part of the reason is that +3 to one test is superior to +1 scattered over 3 tests. It's just much easier to benefit from the former.
Of course, then there's the problem that you only get that full bonus in a small minority of situations. A lot of the time, you'll actually get way less than that. After all, more often that not, you're not spending every action of a turn taking skill tests. And even if you do plan to spend every action taking skill tests, you're not always going to get the Will test you need. And even if you get a Will test, passing Will treacheries is really hard, and Intrepid's +1 is certainly not going to make you likely to succeed on its own. On Expert you would have to add an enormous amount of additional boosting or face a very strong chance of a wasted card.
I think this card is significantly better if there's a Mystic in the party. Mystics proactively test Will all the time and usually build their decks so they have a pretty easy time passing those tests. So you can have the Mystic go first, chuck this onto one of their spells, and then take your turn knowing you'll have the Intrepid bonus. This is much better. Even then, though, I wouldn't take this card. The payoff simply isn't big enough.
Yes, you could combine this with Leo De Luca or Police Badge or whatever, or "Let me handle this!" to try to hunt for a Will test when you want one, but honestly the payoff of this card is just way too low to make these combos interesting for me. I just don't see much here.
Fortunately, the other Guardian card in the pack, Custom Ammunition, is a huge hit, so Guardian players needn't feel too much regret about this one.