The L'isolani RepublicEdit
Initially, the two appear to be on good terms but it is soon proven that Donatello was simply using him and his ability for his own personal gain. It is unknown whether Donatello actually cared for him or not but he seemed to look uneasy or pained during Amadeo's execution. However, it is likely there could be some other cause behind his expression.
Donatello viewed him as a threat against his position as the Doge of L'isolani and so, didn't hesitate to have him executed for the sake of the benefits it brought. That being said, he seemed to respect him and his ability to some level.
The Venedik RepublicEdit
Donatello decided to side with Antonio and Venedik as that was the best possible outcome. It is unknown how he actually feels about him and while he seems respect him and his position, he views him as a rival for the position as the ruler of Centro. It is possible that he doesn't personally care at all for Antonio and is simply after his political power.
The two seem to be familiar with each other, as Donatello causally addresses him during their meeting in L'isolani. Judging from the conversation he has after his meeting with Silvestro (in the manga, with Amadeo), he doesn't find Silvestro particularly pleasant.