For children: - Child aged 12-17 years at time of enrollment (inclusive) - BMI ≥ 95% for age and sex based on CDC growth charts - Comfortable speaking and reading English (due to limitations in intervention materials and qualitative data collection tools) - Child is an active patient at Tufts Medical Center - Child can provide assent, Willing and able to participate in the 18-month study, including attendance at scheduled in-person or telehealth sessions - Access to a device with internet capability (e.g., computer, tablet, or smartphone) to engage with online intervention For parents or guardians: - Age 18+ at time of enrollment - Primary caregiver is the legal guardian of the child - Comfortable speaking and reading English (due to limitations in intervention materials and qualitative data collection tools) - Can provide consent, Willing and able to participate in the 12-month study, including attendance at scheduled in-person or telehealth sessions - Access to a device with internet capability (e.g., computer, tablet, or smartphone) to engage with online intervention components.

For Children: - Participation is medically or developmentally inappropriate, including but not limited to: Severe intellectual disability or cognitive impairment preventing meaningful participation, Active eating disorder diagnosis or concerning disordered eating behaviors, Severe physical limitations or mobility impairments that would limit participation in lifestyle activities, Active unmanaged psychiatric condition (e.g., acute suicidality or psychosis) that may interfere with participation or consent - Planned bariatric surgery or other weight loss procedures during the 12-month study period - Child is currently pregnant or plans to become so in the next 12 months - Plans to relocate or transfer care away from the participating clinic during the 12-month study period - Concurrent participation in another weight management clinical trial that could confound results or interfere with study activities - Parent is unwilling or unable to participate in the study - Child is a parent (has their own child), is in the armed forces, or lives independently and manages their own affairs (emancipated) - Child is a foster child, in state custody, or has an open DCF case pending as identified in the electronic medical record or disclosed verbally by child or guardian. For parent: - Severe intellectual disability or cognitive impairment preventing meaningful participation - Plans to relocate or transfer care away from the participating clinic during the 12-month study period - Unwilling or unable to participate in the study.