We discuss a covariant extension of interactions between scalar fields and fermions in a flat spacetime. We show, in a covariant theory, how to evade fermionic ghosts appearing because of the extra degrees of freedom behind a fermionic nature even in the Lagrangian with first derivatives. We will give a concrete example of a quadratic theory with up to the first derivative of multiple scalar fields and a Weyl fermion. We examine not only the maximally degenerate condition, which makes the number of degrees of freedom correct, but also a supplementary condition guaranteeing that the time evolution takes place properly. We also show that proposed derivative interaction terms between scalar fields and a Weyl fermion cannot be removed by field redefinitions. * K = A ab B aβ,J B aβ,J C αb,I D αβ,IJ D αβ,IJ Cα b,I Dα β,IJ Dαβ ,IJ = Lφ aφb −Lφ aψ β J −Lφ aψβ J Lψα Iφ b Lψ α Iψ β J Lψ α Iψβ J Lψα Iφ b Lψα Iψ β J Lφ a φ b −Lφ a ψ β J −Lφ aψβ J Lψα I φ b Lψα I ψ β J Lψ α Iψβ J Lψα I φ b Lψα I ψ β J Lψα Iψβ J δφ b δψ β J δψβ J . (5)From the definition of the canonical momenta of fermions, we generally haveafter we locally solve the canonical momenta of the scalar fields forφ a , which can be justified by our assumption that ∂π a /∂φ b = A ab has an inverse. Taking the derivative of (B1) with respect toψ β J andψβ J , we have Lψ α Iψ β J = Lφ aψ β J ∂F α,I ∂π φ a + ∂F α,I ∂ψ β J ⇔ ∂F α,I ∂ψ β J