Surprisingly love Grendel (also has a cute ant head), really fun frame to play and while he isn't going to be doing any endless missions, he just generally has a great mechanic. There are some issues and QoL I'd like to point out:
1) Mostly fine, although it could use a slight boost on the "fullness" threshold, from five to seven.
2) Switching buffs is clunky, either automatically cast buffs that haven't been cast followed by the lowest time ones when available or cast all three buffs when held and a random one on press.
3) Holding the key roots Grendel but gives him a large armor buff based on how many enemies are in his stomach, which also powers up a shot that consumes more and more eaten enemies, which then shoot out like a tight shotgun blast.
4) Update "step height" so it rolls through doorways that open up and down, preventing from being stopped by the lower door. This will also solve other "low" obstacle problems. Also maybe allow syandanas to hover stick to the back of the ball for coolness.
As for his passive, allow him to gain a few points of armor for every enemy that actually dies in his stomach. It still isn't going to make him as tanky as Inaros or any of the other 90% DR frames but he'll get closer to it by actually using his skillset.