\documentclass[tikz,border=2mm]{standalone}
\begin{document}
\begin{tikzpicture}
\draw (0,0)--(5,0) node[pos=.8, below]{1} node[below right]{$q$};
\draw (0,0)--(0,5) node[pos=.8, left]{1} node[above left]{$r$};
\draw (0,4)-|(4,0) node[pos=0.125, above] {$r^\star(q)$} node[pos=0.625,right]{$q^\star(r)$};
\draw[ultra thick, red, dashed] (0,0)--(2,0)|-(4,4);
\draw[ultra thick, red, dashed] (0,4)--(0,2)-|(4,0);
\node[below=5mm] at (0,0){(Tails)};
\node[below=5mm] at (4,0){(Heads)};
\node[left=5mm] at (0,0){(Tails)};
\node[left=5mm] at (0,4){(Heads)};
\end{tikzpicture}
\end{document}