What do you call a word that takes the place of a noun in a sentence?

What pronoun can take the place of the noun in the sentence, 'Katrina never gets tired of assisting and helping others.'?

What pronoun can I use if I want to refer to my self alone?

What pronoun will best complete the sentence, " _____ know our rights and we are not afraid."

What pronoun best completes the sentence, " I love Randy and I will do my best to make ______ happy."?