Take a peek in the People Application settings. All Messaging contacts are derived from People, so all it can show you is what People is allowing pretty much.
(I would check but I sent my One back, I didn't have it long enough to remember the People settings very well)
Within Sense 4, 'Frequent' is a categorized Smart group within People. There is a Group edit function (look under edit, menu/stack overflow/3-dots, or try long pressing for a context menu) which allows you to limit the number of contacts that can be displayed in the Frequent group.
In Sense 4 you can't set it to 0, but it lets you take it down to 1.
I think if Sense 5 has some similar setting you will be more satisfied in seeing 1 typical contact there rather than 4 random ones.
If HTC removed this edit ability, forgot it, or if your carrier changed People in some way, you might be able to contact HTC and kindly request that it be restored in the next update of Sense...