Social-emotional learning (SEL), character education, and positive behavior intervention and supports (PBIS) are common approaches to Tier 1 universal social, emotional, and behavioral (SEB) supports in schools. There has been emerging evidence supporting the superior effect of integrated prevention that combines approaches to Tier 1 universal SEB programming compared to the traditional fragmented piecemeal approach. We conducted a cluster randomized trial to examine the additive effect of "PurposeFull People" ("PfP;" an SEL and character education program) in the context of schools implementing Tier 1 PBIS with fidelity. This study took place in five elementary schools implementing Tier 1 PBIS with fidelity. Twenty teachers/classrooms were randomly assigned to treatment ("PfP" combined with Tier 1 PBIS; n[subscript treatment] = 10) or control condition (Tier 1 PBIS only). An average of eight students were randomly recruited from each teacher's classrooms (n[subscript student] = 161; n[subscript treatment student] = 81). Four student SEB outcomes (behavior expectations, behavior discipline, character, and academic engaged time) were assessed at baseline and 4-month post-test. Multilevel ANCOVAs and generalized linear mixed models were used to assess the effectiveness of "PfP" and the effect variation across subgroups (i.e., student demographics and baseline status). Compared to PBIS alone, "PfP" combined with PBIS led to significantly larger improvements in students' behavior expectations, behavior discipline, character, and academic engaged time. Cross-level interaction results indicated that the effectiveness of "PurposeFull People" on students' SEB outcomes varied based on their baseline status, where students struggling most at baseline demonstrated the largest improvement. Limitations and implications for future research and practice are discussed.