Complex sentence: At Kensington, he befriended his Welsh body-servant, Jenkin Lewis, whose memoir of the Duke is an important source for historians, and operated his own miniature army, called the "Horse Guards", which eventually comprised 90 boys.
Split into simpler sentences: At Kensington, Gloucester befriended his Welsh body-servant, Jenkin Lewis, whose memoir of the Duke is an important source for historians. With Lewis, militarily minded Gloucester operated his own miniature army, called the "Horse Guards", eventually comprising 90 boys.