The growing demand for personalized human–computer interaction calls for methods that jointly model emotional states and personality traits. However, large-scale multimodal corpora annotated for both tasks are still lacking. This challenge stems from integrating diverse, task-specific corpora with divergent modality informativeness and domain characteristics. To address it, we propose SSL-MEPR, a semi-supervised multi-task cross-domain learning framework for Multimodal Emotion and Personality Recognition, which enables cross-task knowledge transfer without jointly labeled data. SSL-MEPR employs a three-stage strategy, progressively integrating unimodal single-task, unimodal multi-task, and multimodal multi-task models. Key innovations include Graph Attention Fusion, task-specific query-based cross-attention, predict projectors, and guide banks, which enable robust fusion and effective use of semi-labeled data via a modified GradNorm method. Evaluated on MOSEI (emotion) and FIv2 (personality), SSL-MEPR achieves a mean Weighted Accuracy (mWACC) of 70.26 and a mean Accuracy (mACC) of 92.88 in single-task cross-domain settings, outperforming state-of-the-art methods. Multi-task learning reveals domain-induced misalignment in modality informativeness but still uncovers consistent psychological patterns: sadness correlates with lower personality trait scores, while happiness aligns with higher ones. This work establishes a new paradigm for extracting cross-task psychological knowledge from disjoint multimodal corpora, demonstrating that semi-supervised multi-task cross-domain learning can bridge annotation gaps while preserving theoretically grounded emotion–personality relationships.