I distinctly remember the first time that I heard this version of Relentless' theme song - One night during my freshman year in college, a neighbor of mine came bursting into my room with his newly bought copy of the game and excitedly told me to pop it into my CD player so that we could have a listen. I didn't really talk to him that much beforehand but somehow he guessed that I knew about Twinsen's Adventure. I had previously played the floppy disk version and knew how good the theme song was, so I didn't hesitate to comply with his request. At the time, I thought that the theme was already impressive despite having only listening to it in midi form. However, after hearing the real thing, I knew that I had to get myself my own copy of the CD version as well.
What was unique about Relentless: Twinsen's Adventure (or Little Big Adventure depending on where you live) was the ability to switch between four different behavioral modes - normal, discreet, aggressive, and athletic. In normal you could walk around, mess with objects, and talk to people. Discreet had you sneaking around ala your typical stealth game. Aggressive, on the other hand, gave you the ability to beat the crap out of anyone, including the good guys, which was the best part of the game for many. And if you just wanted to avoid things, you could switch to Athletic mode for running. There were already stealth games back in the NES days, but as far as 3D stealth games are concerned, this was one of, if not the, earliest (for reference, Metal Gear Solid came out in 1998 while Relentless came out in 1994). It's too bad that the company that was responsible, Adeline, no longer exists as a sequel or remake with updated controls would be a great addition to any download service.
As far as I know, Relentless is abandonware, so you can download and play it for free without worry. Check out Little Big Adventure HQ to get both Twinsen's Adventure and Twinsen's Odyssey. The site has details on how to install the games on most PCs, so you should be able to play it no matter how old or new your system is.