The purpose of this study is to evaluate how well LY4213663 is tolerated and what side effects may occur in healthy participants and participants with rheumatoid arthritis (RA). The study drug will be administered either subcutaneously (SC) (under the skin) or intravenously (IV) (into a vein in the arm). Blood tests will be performed to investigate how the body processes the study drug and how the study drug affects the body. The study will last approximately 33 weeks excluding screening.