A Study of LY4213663 in Healthy Participants and Participants With Rheumatoid Arthritis

Inclusion Criteria: * Have a body mass index (BMI) of 18.0 to 32.0 kilogram per square meter (kg/m²), inclusive Part C and D Only: * Have a body weight at least 50 kilograms and BMI within the range 18.0 to 40.0 kg/m² (inclusive) at screening * Have a diagnosis of adult-onset RA for at least 3 months prior to screening Exclusion Criteria: Healthy Participants for SAD Part A and MAD Part B Only: * Have a history or presence of cardiovascular, respiratory, hepatic, ophthalmological, renal, gastrointestinal, endocrine, hematological, neurological, or psychiatric disorders capable of significantly altering the absorption, metabolism, or elimination of drugs Participants with RA for Parts C and D Only: * Have Class 4 RA according to American College of Rheumatology (ACR) revised criteria * Have a 12-lead electrocardiogram (ECG) abnormality at screening * Have a current or recent acute active infection