Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
This is the detection for a family of Trojans that operates as a backdoor allowing remote access, but they are also capable of using you computer for bitcoin mining, for click fraud and to hijack the victims search results.
There are several ways this Trojan can land on a system:
Malwarebytes can remove Trojan.Sefnit without further user interaction. There could be other malware present if this Trojan has been active.