  Graphene nanoribbons are a promising candidate for fault-tolerant quantum
electronics. In this scenario, qubits are realised by localised states that can
emerge on junctions in hybrid ribbons formed by two armchair nanoribbons of
different widths. We derive an effective theory based on a tight-binding ansatz
for the description of hybrid nanoribbons and use it to make accurate
predictions of the energy gap and nature of the localisation in various hybrid
nanoribbon geometries. We use quantum Monte Carlo simulations to demonstrate
that the effective theory remains applicable in the presence of Hubbard
interactions. We discover, in addition to the well known localisations on
junctions, which we call `Fuji', a new type of `Kilimanjaro' localisation
smeared out over a segment of the hybrid ribbon. We show that Fuji
localisations in hybrids of width $N$ and $N+2$ armchair nanoribbons occur
around symmetric junctions if and only if $N\pmod3=1$, while edge-aligned
junctions never support strong localisation. This behaviour cannot be explained
relying purely on the topological $Z_2$ invariant, which has been believed the
origin of the localisations to date.
