I've once used so-called 'Danish Oil' (Watco brand) to finish a walnut handle on an Opinel of mine, after I'd reshaped it. It's a hard-curing finish, and works pretty well to seal the wood. Apply it in maybe 3 very, very light & thin coats, wiped on with a clean rag (old cotton t-shirt works well) and allowing it time to dry thoroughly between each coat (I'd give it a few hours at least, or up to a full day, depending on local humidity). If it's drying or curing very slowly, you'll notice a tacky feel to it when you touch it. Sanding the wood before doing so might be a good idea, maybe up to something like ~320 or so. As stripped of color and (probably) moisture as that handle looks, you might notice the wood drinking up a lot of that Danish oil. Even so, be very careful about over-applying it, as it'll get too heavy to cure completely (leaves a soft & gummy mess which may never fully harden).
I've since reshaped and refinished the handle on that same knife again (just to thin the handle some more; no issues with the finish itself, as it was). 2nd time around, I used a different finish and a more tedious method. But here are a couple pics of how it looked after the Danish Oil treatment, which was
very easy to get done, which is why I suggest it here. I used the clear (unstained) finish, first sanding the bare wood up to maybe ~800 - 1000 or so (maybe overkill, past a certain point).
You can get the Danish oil in stain-enhanced 'colors' as well. I think one option is a Walnut finish, which would probably help bring some color back to that faded handle (looks like it could be walnut or maybe some form of rosewood, both common on knives like these). The clear version with no added stain won't darken the wood much, but may add a little amber(ish) tone to lighter woods, when cured.
This is just one suggestion. I'd loiter around a bit more, and wait for others to offer theirs as well. Lots of ways to get it done, so it's good to consider as many options as you can, before diving in.
