Sinclair is a timid young man. He's a young adult that was suddenly thrown into the life of a Fixer—he doesn't have much experience on the field and is uncertain of his own capabilities/judgment.  

When voicing Sinclair, the most important thing to remember is his hesitance. When ellipses (...) occur in his lines, you can trail off your words before continuing to demonstrate that he doesn't have confidence in what he's saying. His tone is fairly soft, with a medium to high pitch and nervous undertones. A slight German accent fits his character, but is unneeded.

Sinclair's lines can be found here.