A composite model of Σ as a bound state of Λπ and NK̄ is developed in terms of dispersion relations for form factors and an approximate Bethe-Salpeter equation. The effective coupling constants of Σ to its constituents are predicted to be 8.7<~|gΣ0,Λπ0|<~10.7 and 6.4<~|gΣ0pK-|<~7.9 for the range 5.0<~|gΛpK-|<~13.5. The relative sign of gΣ0Λπ0 and gΣ0pK- is found to be opposite to the relative sign of gppπ0 and gΛpK-. The model is further applied to a calculation of the on-shell electromagnetic form factors of the ΣΛ system. Under the assumption of ρ dominance of the kaon isovector electromagnetic form factor, the ΣΛ transition moment is determined to be G2 (O)=gΛpK- gΣ0pK- 0.9×10-2 nuclear magnetons, whereas the form factor which would vanish in the limit of equal Σ0 and Λ masses is two orders of magnitude smaller than the transition moment.