\documentclass{article}

\usepackage{agda}

\DeclareUnicodeCharacter{9711}{$\bigcirc$}

% The ``Missing character: There is no ◯ in font Asana Math'' warning
% can be fixed by changing font, e.g.:
%
% \setmathfont{XITS Math}

\begin{document}

\begin{code}
-- ◯ = Set
◯ = Set
\end{code}

\end{document}