  Graphene can develop large magnetic moments in custom crafted open-shell
nanostructures such as triangulene, a triangular piece of graphene with zigzag
edges. Current methods of engineering graphene nano-systems on surfaces
succeeded in producing atomically precise open-shell structures, but
demonstration of their net spin remains elusive to date. Here, we fabricate
triangulene-like graphene systems and demonstrate that they possess a spin
$S=1$ ground state. Scanning tunnelling spectroscopy identifies the fingerprint
of an underscreened $S=1$ Kondo state on \rev{these} flakes at low
temperatures, signaling the dominant ferromagnetic interactions between two
spins. Combined with simulations based on the meanfield Hubbard model, we show
that this $S=1$ $\pi$-paramagnetism is robust, and can be manipulated to a
$S=1/2$ state by adding additional H-atoms to the radical sites. \rev{Our
results demonstrate that $\pi$-paramagnetism of high-spin graphene flakes can
survive on surfaces, opening the door to study the quantum behaviour of
interacting $\pi$-spins in graphene systems.
