"He's very personable, very engaging, very compassionate," O'Connell said.
Sean O'Connell, 38, had been with the patrol for 16 years and had previously been named Trooper of the Year for his district.
The family has been mourning privately, but O'Connell wanted to speak to the media before Thursday's memorial at Everett's Comcast Arena.
"His greatest strength was being able to open up a conversation with somebody who was a complete stranger and connecting with them," O'Connell said.
Sean had multiple sclerosis and Fran said that if anyone wanted to make donations in his brother's honor, they could give to the National MS Society or the WSP Trooper Fund.
O'Connell read a short statement to the media, saying the family wanted to honor his brother by being more like his brother and to share his warm nature with everyone.
"We will answer our calling, love our friends and family, and find humor in the everyday events of life," he said.