My servant. Three are so called in this book: David (here); Israel or Jacob (the nation) (Isa_41:8; Isa_42:19; Isa_43:10; Isa_44:1; Isa_45:4; Isa_48:20; Isa_49:3 and whole chapter); and Messiah (Isa_42:1; Isa_65:8).
David's sake. Note how Hezekiah refers to these words in his "Songs of the Degrees" (Psa_132:1, Psa_132:10).