Summary
Kleber is more athletic than he gets credit for, as he isn't the quickest defender laterally but can survive on the perimeter and can elevate decently to protect the rim. He isn't a great defender in space but he's not a turnstyle either, and overall he's a really smart help defender.

While he has looked like he has lost a little bit of athleticism this season, and he's now in his thirties, he played really well in the playoffs, having some big shooting games and often being in the closing lineup as a small-ball center. Kleber has some rim-protection ability, as his block rate is decent and he demonstrates good verticality to contest shots at the rim.

Kleber mostly plays the four, but does have some ability to be a stretch-five, and has played at center an increased amount in '21-'22. After his shooting percentages rose steadily from 2017-2021, he was down a bit in '21-'22, only hitting 33% from three after shooting a career high 41% from three in '20-'21. However, he did hit 44% of his threes in the playoffs. Kleber can't do too much else offensively other than hit spot-up shots, but as a role-player he isn't asked to do much more than that.

Based on how Kleber was able to be in the closing lineup for the Mavs in the playoffs by hitting enough shots offensively and defending at a high-level, I wouldn't expect the Mavs to waive him, so I don't expect him to actually hit free agency until 2023. If they guaranteed his contract, he would likely still have positive trade value.
 
Cap Considerations
Kleber's $9.2 million salary for 2022-2023 is non-guaranteed, and if he is waived he will become an unrestricted free agent and the Mavs will have no form of Bird rights on him.

If Kleber becomes a free agent, other teams will have multiple ways to sign him as he likely could be had for the Non-Taxpayer MLE, or possibly even the Taxpayer MLE.