Let them hear once again the strength, courage, and capabilities you see in them, often below the surface, but very much there.
I think "very much there" means being close to the surface.
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"Often below the surface, but very much there."
The phrase "very much there" is used to tell the reader that though the qualities of strength, courage and capabilities are often hidden or dormant or just around the corner, they're certainly still there. Perhaps it may take a while for those qualities to show up or may be a circumstance is required to push them forward.
For example, His loving nature is overshadowed by his temper, but it is still very much there. This implies that the person has a temper which sometimes makes him unapproachable but if you look closer, you will certainly be able to see that he is loving in nature.
Hope this helps. NS
In this context, very much there emphasizes how accurate it is that those with the capabilities mentioned in the sentence really do have them. (See Macmillan Dictionary)
The author apparently felt the need for this emphasis, because the capabilities are not always obvious or visible, hence often below the surface.